Introduce module coalesce

[phimuemue]

Hi! As #440 (thanks for reviewing and merging, btw) presumably conflicts with any existing PRs touching coalesce et al, I thought we could just as well seize the opportunity and move these types into an own module (so to speak as a starting point for the problem mentioned in #423 (comment)).

I cut-pasted Coalesce et al into an own module, rearranged the code so that specialized types follow their generic variants, and ran cargo fmt on the new module.

I suggest to import the coalesce types into adaptors similarly to what we already have for multi_product.

[jswrenn]

bors r+

[bors]

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