Allow constants and instance / class variables as receivers for setter proc pointers

[HertzDevil]

Currently, the following proc pointers are allowed:

->foo=(Int32)   # implicit receiver
->x.foo=(Int32) # `x` is a variable

This PR adds support for the following proc pointers, which currently produce Error: unexpected token: =:

->Foo.foo=(Int32)   # path (type or constant)
->@foo.foo=(Int32)  # instance variables
->@@foo.foo=(Int32) # class variables

Operators that end with =, most notably []=, are unaffected as they are already supported (as long as there is a receiver).