Prune unnecessary system header includes from upcxx.hpp

<upcxx/upcxx.hpp> currently has the side-effect of including some headers like <cassert>, <cmath> and <climits>, none of which are used by the implementation and just clutter up the user programs's namespace (most notably the macro namespace) for no good reason.

It's deliberately unspecified what system headers are pulled in by <upcxx/upcxx.hpp>, but we should make an effort to minimize the set to things that are actually necessary.