disallow implicit return if result is used, don't use void context

[metagn]

fixes #16855

Assigning to the result variable makes the assignment expression have a type of typed, which causes any subsequent expressions in the same statement list as the assignment to require a void type, so that the result variable cannot be overriden by implicit returns. This has multiple issues:

1. This happens for any statement list containing an assignment to result, not just the proc body.
2. Because this only triggers for direct assignments to the result variable, complex assignments like foo(result) can fool this and still allow an implicit return.

To fix this, we track if the result symbol is used, then error if an implicit return is attempted. An exception is granted to return statements which generate an assignment to the result variable: if the result variable was not used before, it will not count as used after.

[metagn]

Bugs aside, mixing these is pretty common: https://github.com/arnetheduck/nim-results/blob/71d404b314479a6205bfd050f4fe5fe49cdafc69/results.nim#L1545-L1564

Something like varpartitions might be overkill, maybe a simpler version is to keep enforceVoidContext but opt it in to the expression context of the proc body with a TExprFlag, which I think makes sense but foo(result) wouldn't be fixed.