Order[A] now implements eqv() in terms of compare().

Previously, we left eqv() unimplemented. The rationale behind that no
longer makes sense, so now we provide an implementation and require
the user to override it if desired.

This change ended up touching every file that defines an Order or Eq,
but fortunately the required changes were minor.
Move log from NRoot to Trig
specialize on Byte/Short, clean up a bit
Remove unused Conversions support
Remove Gaussian and EuclideanRational from spire.math