Require GMP version 5.0.5
Add mpfp, a wrapper for MPFR
Add numeric_traits for double and float
Add test for mpfp
Update util/numerics/CMakeLists.txt to add mpfp, double, float
Update interval
Add => as a primitive. Define Not, And and Or using =>. Add MP and Discharge as axioms.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>