Make make_const() check the size and precision of a T_Float Value,

and produce either FLOAT8 or NUMERIC output depending on whether the
value fits in a float8 or not.  This is almost back to the way the
code was before I changed T_Float, but there is a critical difference:
now, when a numeric constant doesn't fit in float8, it will be treated
as type NUMERIC instead of type UNKNOWN.