macro type class and is_x functions for closure types
got expand-1 working for unhygenic macros. i think.
fixed stupid bug in evaluating `unquote`
hacky implementation of `expand`
refactored macro expansion
refactored apply
removed commented out code
removed REPL help command
renamed get_default_env to get_builtin_env
cleaned up unused imports / formatting
rewrote parse_file function