Add Crystal::Macros::ProcNotation#resolve and #resolve?

[HertzDevil]

ProcNotations represent type names, so it makes sense that they should have the #resolve and #resolve? macro methods. In particular, they may appear in TypeDeclarations, so the following is now allowed:

macro foo(node)
  {{ node.type.resolve }}
end

foo(x : Int32 -> String) # => Proc(Int32, String)