get rid of recurse/overdub cycle in favor of only overdub

[jrevels]

supercedes #62
fixes #59 (@Keno)
fixes #61

In my ongoing quest to increase user confusion, this PR removes the current Cassette.overdub and changes the name of Cassette.recurse to Cassette.overdub. This new overdub function performs a similar operation to the old recurse, but now manually inlines the body of the old overdub function instead of calling such a function.

This is a fairly disruptive change this close to Cassette's impending initial release, but it's worth it. This approach has drastically less reliance on splatting optimizations than the current approach, and additionally ensures there are (mostly) no more unspoofed recursion cycles left to trigger inferences limiting heuristic. As a result, many things are inferring now that weren't before (including the examples from #59). As a nice little side effect, we get to cut down our API by a function.

The tradeoff is that this approach requires more IR-munging than the old approach. Lucky, I was able to move most of the finnicky stuff to a reusable utility function (insert_ir_elements!).

All tests here pass locally. I'm not sure if CI will pass immediately, though, since this requires JuliaLang/julia#28279 which was just merged.

Oh, and this removes Fallback (which as I've said before, was kind of just sitting around as an idea, but I don't think it's very useful here).

[jrevels]

tests pass using latest Julia, CI just didn't have an updated build, so I'm merging