rewrite `asyncmap` to use plain buffers instead of `Ref` (#61445)

fixes https://github.com/JuliaLang/julia/issues/60304,
https://github.com/JuliaLang/julia/issues/61440,
https://github.com/JuliaLang/julia/issues/61441

the old approach was a little "clever" in this map-twice approach to
first make an array of `Ref` and then fill those results. I imagine the
intention was to use all the `map` specializations to keep the same
output type whenever possible. but `collect_similar` can also do this
and avoids the bugs arising for arrays where `x[i] !== x[i]`.

also there were several paths in the mutating `asyncmap!` that were
completely non-functional and some other drive-by cleanups.

co-developed with Opus 4.6