Add '__int_to_char' builtin.
Prefer safety. Add unsafe functions just in case.

* Make 'head' and 'last' return option types.

* Use 'option' instead of 'if' with 'isEmpty'.

* Add 'unsafeHead' and 'unsafeLast' with old behavior.

* Rename '#!' to '#=' and '##!' to '##='.
Add 'find'.