Use hygienic identifier to replace `self` in closure body

When I wrote !5 (merged), I wasn't aware that hygiene in proc macros was stable. It turns out that it is (sort of, via mixed_site), so use a hygienic this variable instead of a non-hygienic one with underscores.

This also updates some comments and function names from !5 (merged) that were incorrect. They described a prior version of the MR that attempted to pass destructured arguments to the closure explicitly.