Remove mkSingleton from the API (#5366)

Remove mkSingleton from the API (#5366)

This PR removes mkSingleton from the API after removing subtyping from set theory and introducing mkInteger to the API.
Instead the user needs to use Solver::mkTerm(api::SINGLETON, element) where element has the right type.
Internally NodeManager::mkSingleton(type, element) is still needed to determine the type of the set.
Other changes:

Renamed Solver::mkTerm(Op op, .. functions to Solver::mkTermFromOp(Op op, ...
Added mkTermFromOp to the python API