use cbuilder for tuple/object generation

[metagn]

based on #24127

Needs some tweaks to replace the other struct type generations, e.g. seqs, maybe by exposing BaseTypeKind as a parameter. C++ and codegenDecl etc seem like they are going to need attention.

Also Builder should really be distinct string that one has to call extract on, but for this to be optimal in the current codegen, we would need something like:

template buildInto(s: var string, builderName: untyped, body) =
  template `builderName`: untyped = Builder(s)
  body

buildInto(result, builder):
  builder.add ...

but this could be a separate PR since it might not work with the compiler. The possibly-not-optimal alternative is to do:

template build(builderName: untyped, body): string =
  var `builderName` = Builder("")
  body
  extract(`builderName`)

result = build(builder):
  builder.add ...

where the compiler maybe copies the built string but shouldn't.

[Araq]

Also Builder should really be distinct string that one has to call extract on ...

Correct, but this will be done much later, the goal for now is tiny refactorings that keep the CI clean at all time until we're confident enough that we can replace the implementation with something else.

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 2cdc0e9

Hint: mm: orc; opt: speed; options: -d:release
174567 lines; 8.368s; 654.641MiB peakmem