Write to Future.internalValue directly in macro-translated code

[arnetheduck]

Although #418 was ultimately not merged, it contains an idea to improve performance in general: instead of allocating a separate var result that gets injected, it instead injects a template result = fut.internalValue which means we use the in-flight future as temporary storage for results instead of a local variable thus saving memory, zero:es and copies.

The technique is interesting but needs to be tested against nim-lang/Nim#22605 of which it may run afoul (along with other correctness / observability issues).