Remove concrete floating-point inits from integer types (#19624)
Also removes .gyb from FloatingPoint.swift, since we barely are using it after this change.
Rework Self._convert from integer, by making it require RawSignificand: FixedWidthInteger. This requirement should have always been there, and the existing implementaiton wouldn't have actually worked correctly without it. Making it explcit makes the implementation quite a bit simpler, which is nice.
Additionally add a fast-path conversion that will catch all concrete integer types without needing to be a concrete implementation itself. Room for further improvement, but good start. See RandomDouble performance improvements, for example.
