math/big: handle NaNs in Float.Cmp

math/big: handle NaNs in Float.Cmp

Also:
- Implemented NewFloat convenience factory function (analogous to
  NewInt and NewRat).
- Implemented convenience accessors for Accuracy values returned
  from Float.Cmp.
- Added test and example.

Change-Id: I985bb4f86e6def222d4b2505417250d29a39c60e
Reviewed-on: https://go-review.googlesource.com/6970
Reviewed-by: Alan Donovan <adonovan@google.com>