Cleanup return type open syms

[Menduist]

Fixes status-im/nimbus-eth1#963
For some reason, the code was generating this kind of AST for the return type:

    Call
      OpenSymChoice
        Sym "[]"
        Sym "[]"
        ...
      Sym "Future"
      Call
        OpenSymChoice
          Sym "[]"
          Sym "[]"
          ..
        Ident "seq"
        Ident "OpenObject"

And this "recursive" OpenSymChoice wasn't handled properly by the macro

[mratsim]

I'm unsure how often we stumble upon this but maybe we need a toUntypedAST proc in stew to cleanup all the situation where we end up receiving typed AST in an untyped macros?

For example, I'm sure we hit this in serialization libraries macros. I didn't test using taskpools in a generic proc or a template yet but it's very possible as well.

[Menduist]

It would be a bit overkill for this fix, since we can only have OpenSyms AFAICT, but sure would be good to have as a general helper
and this kind of bugs are hard to find

[Menduist]

nim-lang/Nim#11091 (comment)
Apparently we could also put it in the stdlib

[Araq]

The fix is acceptable but I would do it differently. I would use an extractReturnType helper instead. Your patch is conceptually quite brutal -- you change the AST to something else and then you extract what you're interested in.

[Menduist]

Not sure I understand @Araq, is it because I modify the AST in place instead of copying it?
If so, last commit fixes that

[mratsim]

In my experience working with types with macros is fraught with peril nim-lang/RFCs#44 and worse, this might manipulate generics which tend to become a timesink status-im/nimbus-eth2#2219

If we can avoid manipulating types in macros and just modify AST to make it canonical untyped, it will likely save hours of debugging afterwards at the cost of forcing the compiler to redo semcheck/sigmatch.