mp_integer: bits of code for the new int promotion. Not functional or tested yet.
type_traits: replace boost integer_traits with std::numeric_limits.
type_trait: add small helper that determines a symmetric range for a signed integer type
for which it is safe to take +/-.
mp_integer: more work and cleanup on the new approach to int promotion.

Also restored mp_integer<> as coefficient type in the gastineau tests.
mp_integer: tweaks, checks and tests for the static view class.