Draft: Use a uniform return convention in bytecode for unary results

This MR fixes #22958 (closed) by implementing a uniform return convention for lifted and unlifted values in bytecode, excluding unboxed tuples, which are special. The patch is fairly small, and it includes some other nice simplifications to parts of the bytecode compiler and interpreter that needed to work around the nonuniform calling convention.

I’ve marked this MR as a draft for now because I would like to add some comments to the implementation to make it a little easier to understand, but otherwise the patch should be complete.