Rewrite inductive-cons as a special-form; rename to datacons

* src/lparse.ml (infer): Remove Pcons case.
(sform_datacons, sform_type): New functions.
(register_special_forms): Name the initialization code.  Use the two
new sform_* functions.
(dynamic_bind): New function.
(default_lctx): Use them.

* src/pexp.ml (pexp): Remove Pcons.

* src/lexp.ml (pdatacons): New var.
(lexp_unparse, lexp_unparse): Use it.