Fused multiply-add (Base.fma) support for Num

[nsajko]

Currently trying to use fma on Symbolics variables (Num) fails. This should be supported, I think.

Given that Num can contain either symbolic or numeric values, fma for Nums can behave differently depending on whether some arguments are symbolic:

1. Delegate to muladd if any of the three Num arguments to fma is symbolic.
2. Otherwise (if all of the arguments are numeric) use fma on the unwrapped numeric values and return a Num that wraps the result thus obtained.

[ChrisRackauckas]

I would think this just needs to get registered.