Be more strict about ProcNotation variable declarations

[HertzDevil]

The following is an error:

x : -> Int32.class = ->{ Int32 } # Error: type must be Proc(Int32), not Proc(Int32.class)

Here the Int32.class is already read as a type expression. This PR removes an incorrect conversion to Int32.class's instance type.

This PR also now disallows the following ProcNotations, because they would have been rejected if they were rewritten into their equivalent Proc instances:

x : -> Reference # ditto for similar base types like Object and Number
x : Array ->     # uninstantiated generics
x : -> ARGV      # constants

No Procs with these argument types could possibly be constructed, so this shouldn't be a breaking change. Resolves part of #11371.

[sdogruyol]

Thank you @HertzDevil 🙏