Intinf mod

Changes intended to reduce the GMP library awareness of int-inf.sml to just those exposed by MLton.IntInf. The sign function uses less-than tests directly; an alternative approach would be to expose the default integer type to int-inf0.sml and implement sign: int -> Int.int in the implementation-aware part of the interface.

Removed isNeg from the PRIM_INT_INF interface as well - unlike its companion functions it could only be called on Big representations, it was only used to implement sign, and this seemed error-prone.