Initial Library Support

src/builtins/functions.rs

src/builtins/mod.rs

src/lib.rs

src/shell/binary.rs

src/shell/directory_stack.rs

src/shell/flags.rs

src/shell/flow.rs

src/shell/flow_control.rs

src/shell/library.rs

src/shell/mod.rs

src/shell/pipe_exec/job_control.rs

src/shell/status.rs

src/shell/variables/mod.rs