Linked unboxed-boxed versions of types via # paths

[rtjoa]

Change the typechecking of pairs of types like float and float# so # acts as a postfix operator on paths that accesses the unboxed version of a type. We make this change in preparation for implicit unboxed records (which follow in #3527).

Observable effects:

1. Aliases of predef types with unboxed versions also get unboxed versions.

type t = float
type u : float64 = t#

2. Shadowing the boxed version of a type also shadows the unboxed version.

type float
type t = float#
(* Error: "float" has no unboxed version. *)

3. Some error messages change slightly.

Reviewing

To minimize conflicts with the following PR, some comments are written as if implicit unboxed records already exist.

Stack

1. Linked unboxed-boxed versions of types via # paths #3526
2. Implicit unboxed records #3527

Review: @ccasin