Add support for T::Sig::WithoutRuntime

[Morriar]

This PR introduces a few changes:

• Signatures can be marked as without_runtime
• Signature without_runtime are printed as T::Sig::WithoutRuntime.sig
• Parse signature with T::Sig::WithoutRuntime.sig
• Handle RBS <> RBI translate for T::Sig::WithoutRuntime.sig

I also spotted an error in the parser where random calls to sig like Foo.sig would be parsed as actual Sig instances while it shouldn't. Fixed those.

[egiurleo]

🥳