Feature request: GCD for floating-point

Surprisingly, I can't find _any_ implementation of floating-point GCD for Swift, anywhere.

While there are implementations in other languages - Python has heaps - a lot of them gloss over the important detail of rounding. And for those that don't, porting them to Swift too literally may introduce mathematical errors, depending on how the source language differs from Swift in its handling of reals.

Since this library already has a good GCD implementation for integers (which seems to be a prerequisite for a floating-point variant), perhaps it'd make sense to add a version for `FloatingPoint`?

Start by reading the library’s existing integer GCD implementation and the issue discussion about rounding and Swift’s handling of reals. Define the expected behavior for a FloatingPoint GCD, then validate the chosen semantics against representative rounding cases; done means the new variant is implemented with coverage for those cases.

Written by the indexing model from the issue text.