Proposal: Add "fma" to the RealFloat class

[Levent Erkok]

This proposal is very much in the spirit of the earlier proposal on adding
new float/double functions; for instance see here:
https://mail.haskell.org/pipermail/libraries/2014-April/022667.html

"fma" (a.k.a. fused-multiply-add) is one of those functions; which is the
workhorse in many HPC applications. The idea is to multiply two floats and
add a third with just one rounding, and thus preserving more precision.
There are a multitude of applications for this operation in engineering
data-analysis, and modern processors come with custom implementations and a
lot of hardware to support it natively.

I created a ticket along these lines already:
https://ghc.haskell.org/trac/ghc/ticket/10364

Edward suggested that the matter should further be discussed here.

I think the proposal is rather straightforward, and should be
noncontroversial. To wit, we shall add a new method to the RealFloat class:

  class (RealFrac a, Floating a) => RealFloat a where
      ...
      fma :: a -> a -> a -> a

The intention is that

      fma x y z = x * y + z

except the multiplication and addition are done infinitely-precisely, and
then rounded only once; as opposed to two roundings as one would get with
the above implementation. Most modern architectures directly support this
operation so we can map it easily; and in case the architecture does not
have it available, we can get it via the C-math libraries, where it appears
under the names fma (the double version), and fmaf (the float version.)

There should be no default definitions; as an incorrect (two-rounding
version) would essentially beat the purpose of having fma in the first
place.

While the name "fma" is well-established in the arithmetic/hardware
community and in the C-library, we can also go with "fusedMultiplyAdd," if
that is deemed more clear.

Discussion period: 2 weeks.
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