Implement FRem

[Chobbes]

This operation is unimplemented for both doubles and floats. It seems like flocq doesn't provide a remainder operation by default.

This can currently cause QC tests to fail occasionally, as this operation is generated rarely. We could avoid generating the operation, of course, but we'll need to remember to add it back to the test suite once it's implemented :). It's rare enough that I'm not worried about it right this second.