mp_integer: more tests for the exponentiation method.
mp_integer: make the st/dy members of the int union private and use instead getters with embedded
static status checks.

This would've caught the bug fixed a couple of commits ago.
mp_integer: remove todo item.
small_vector: use safe st/dy getters in the same fashion as we do in mp_integer.
mp_integer: implement absolute value.
mp_integer: add a couple of noexcept specifiers.