Use `small::String` for every string type

Cargo.lock

Cargo.toml

members/braces/Cargo.toml

members/braces/src/lib.rs

members/builtins/Cargo.toml

members/builtins/src/calc.rs

members/builtins/src/conditionals.rs

members/builtins/src/echo.rs

members/builtins/src/lib.rs

members/builtins/src/random.rs

members/builtins/src/test.rs

members/ranges/Cargo.toml

members/ranges/src/lib.rs

members/ranges/src/parse.rs

members/ranges/src/select.rs

src/lib/builtins/command_info.rs

src/lib/builtins/exec.rs

src/lib/builtins/exists.rs

src/lib/builtins/is.rs

src/lib/builtins/job_control.rs