- clarified representation of +/-Inf
- only 0 and Inf values can have 0 precision
- a zero precision value used as result value takes the max precision
  of the arguments (to be fine-tuned for setters)
- the zero precision approach makes Float zero values possible
  (they represent +0)
- more tests

Missing: Filling in the blanks. More tests.

Change-Id: Ibb4f97e12e1f356c3085ce80f3464e97b82ac130
Reviewed-on: https://go-review.googlesource.com/4000Reviewed-by: Alan Donovan <adonovan@google.com>