Remove dependency on the Prim int-inf0 structure so that int-inf.sml need not depend on the specifics of representation. The resulting sign function is potentially slower; a faster sign function could be added to int-inf0.sml at the cost of exposing the default integer to int-inf0.
Remove what appears to be dead code since the 64-bit refactor, int-inf1
Merge pull request #66 from robsimmons/intinf-mod

Changes intended to reduce dependency of `basis-library/integer/int-inf.sml` on GMP-specific details of `basis-library/integer/int-inf0.sml`.