Out of bounds panics on map assignments

Stacktraces and repros looks similar, so I grouped them together.

Reproduction #1:

let map['='] = '='

Panics with: thread 'main' panicked at 'byte index 9 is out of bounds of "'] = '='"', src/libcore/str/mod.rs:2027:9

Stacktrace

stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:385
   6: rust_begin_unwind
             at src/libstd/panicking.rs:312
   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   8: core::str::slice_error_fail
             at src/libcore/str/mod.rs:0
   9: core::str::traits:: for core::ops::range::Range>::index::{{closure}}
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libcore/str/mod.rs:1768
  10: >::unwrap_or_else
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libcore/option.rs:386
  11: core::str::traits:: for core::ops::range::Range>::index
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libcore/str/mod.rs:1768
  12: core::str::traits:: for str>::index
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libcore/str/mod.rs:1633
  13:  as core::iter::iterator::Iterator>::next
             at members/lexers/src/arguments.rs:148
  14:  as core::iter::iterator::Iterator>::next
             at src/lib/parser/assignments/actions.rs:79
  15: ::local
             at src/lib/shell/assignments.rs:167
  16: ::execute_statement
             at src/lib/shell/flow.rs:181
  17: ::on_command
             at src/lib/shell/flow.rs:491
  18: ion_shell::shell::binary::terminate::terminate_script_quotes
             at /home/tibor/git/ion/src/lib/shell/binary/terminate.rs:37
  19: ::terminate_script_quotes
             at /home/tibor/git/ion/src/lib/shell/binary/mod.rs:165
  20: ion_shell::shell::Shell::execute_script
             at /home/tibor/git/ion/src/lib/shell/mod.rs:207
  21: ion::main
             at src/main.rs:54
  22: std::rt::lang_start::{{closure}}
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libstd/rt.rs:64
  23: std::panicking::try::do_call
             at src/libstd/rt.rs:49
             at src/libstd/panicking.rs:297
  24: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:92
  25: std::rt::lang_start_internal
             at src/libstd/panicking.rs:276
             at src/libstd/panic.rs:388
             at src/libstd/rt.rs:48
  26: std::rt::lang_start
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libstd/rt.rs:64
  27: main
  28: __libc_start_main
  29: _start

Reproduction #2:

let map[=]

Panics with: thread 'main' panicked at 'index out of bounds: the len is 4 but the index is 4', members/lexers/src/assignments/keys.rs:67:16

Stacktrace

stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:385
   6: rust_begin_unwind
             at src/libstd/panicking.rs:312
   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   8: core::panicking::panic_bounds_check
             at src/libcore/panicking.rs:61
   9: ion_lexers::assignments::keys::KeyIterator::parse_array
             at members/lexers/src/assignments/keys.rs:67
  10:  as core::iter::iterator::Iterator>::next
             at members/lexers/src/assignments/keys.rs:153
  11:  as core::iter::iterator::Iterator>::next
             at src/lib/parser/assignments/actions.rs:78
  12: ::local
             at src/lib/shell/assignments.rs:167
  13: ::execute_statement
             at src/lib/shell/flow.rs:181
  14: ::on_command
             at src/lib/shell/flow.rs:491
  15: ion_shell::shell::binary::terminate::terminate_script_quotes
             at /home/tibor/git/ion/src/lib/shell/binary/terminate.rs:37
  16: ::terminate_script_quotes
             at /home/tibor/git/ion/src/lib/shell/binary/mod.rs:165
  17: ion_shell::shell::Shell::execute_script
             at /home/tibor/git/ion/src/lib/shell/mod.rs:207
  18: ion::main
             at src/main.rs:54
  19: std::rt::lang_start::{{closure}}
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libstd/rt.rs:64
  20: std::panicking::try::do_call
             at src/libstd/rt.rs:49
             at src/libstd/panicking.rs:297
  21: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:92
  22: std::rt::lang_start_internal
             at src/libstd/panicking.rs:276
             at src/libstd/panic.rs:388
             at src/libstd/rt.rs:48
  23: std::rt::lang_start
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libstd/rt.rs:64
  24: main
  25: __libc_start_main
  26: _start

Build information:

rustc -V: rustc 1.33.0-nightly (4c2be9c97 2019-01-22)

git rev-parse HEAD: 918e1fc4