Now that we depend on C++14, we can use switches in constexpr functions
and no longer need to pass two template parameters.

As part of this refactor some of the CHERI get intrinsics folding code.
This refactoring introduces a minor change to getoffset folding as well:
we no longer fold arbitrary getoffset(setoffset(A, B)) -> B since the
setoffset could have changed the bounds interpretation and therefore
also the result. This optimization is unlikely to make a difference in
practise and is somewhat incorrect so I believe dropping it makes sense.

If we are setting the same value we just read we can simplify the
sequence to just the original argument.