Add new ##... syntax to refer to existing builtins

* src/builtin.ml (lmap): New var.
(add_builtin_cst): New function.

* src/lexp.ml (mkSusp): Don't bother suspending immediates.
(lexp_unparse): Use Pbuiltin.
(_lexp_to_str): Use the ##... syntax.

* src/lparse.ml (macromap): Adjust to earlier change.
(make_macro_map): Use it.
(infer): Handle Pbuiltin.

* src/pexp.ml (pexp): Add Pbuiltin constructor.
(pexp_parse): Recognize new ##... syntax.
(pexp_location, pexp_unparse): Handle it.

* src/prelexer.ml (string_sub): Move to util.ml.