More builtins

- `umask`
- `alias`
- `export`
- `set`
- `unset`
- `shift`
- `eval`
- `which`
- `bg`
- `fg`
- `wait`
- `kill`
- `disown`
- `jobs`
- `true`
- `false`
- `test`
- `source`, a.k.a `.`
- `return`
- `continue`
- `break`
- `getopts`
- `:` (noop, returns 0)
