Remove `ConstantMap<Rational>` (#7635)

Remove `ConstantMap<Rational>` (#7635)

This is in preparation of having two different kinds (CONST_RATIONAL
and CONST_INT) share the same payload. To do so, we cannot rely on
ConstantMap<Rational> anymore to map the payload type to a kind. This
commit extends support in the mkmetakind script to deal with such
payloads by adding a + suffix to the type. The commit also does some
minor refactoring of NodeManager::mkConst() and
NodeManager::mkConstInternal() to support setting the kind explicitly.
Finally, the commit addresses all instances where mkConst<Rational>()
is used, including the API.