Remove plugin support

Cargo.lock

Cargo.toml

src/lib/lib.rs

src/lib/parser/shell_expand/mod.rs

src/lib/parser/shell_expand/words/methods/arrays.rs

src/lib/parser/shell_expand/words/methods/strings.rs

src/lib/parser/statement/splitter.rs

src/lib/shell/assignments.rs

src/lib/shell/binary/readln.rs

src/lib/shell/mod.rs

src/lib/shell/plugins/library_iter/mod.rs

src/lib/shell/plugins/library_iter/redox.rs

src/lib/shell/plugins/library_iter/unix.rs

src/lib/shell/plugins/methods/mod.rs

src/lib/shell/plugins/methods/redox.rs

src/lib/shell/plugins/methods/unix.rs

src/lib/shell/plugins/mod.rs

src/lib/shell/plugins/namespaces/mod.rs

src/lib/shell/plugins/namespaces/redox.rs

src/lib/shell/plugins/namespaces/unix.rs