WIP: Replaced all instances of internal::(U)IntPtr with std::(u)intptr_t. Removed typedef of (U)IntPtr.

Reference issue

Closes issue #2596 (closed).

What does this implement/fix?

Removes an old typedef (regarding use of std::intptr_t vs. std::size_t, see issue #2596 (closed)) used to workaround an issue in ICC. No longer necessary in C++14.

Additional information

Removal of this typedef improves compatibility with the novel CHERI/Morello architecture and should have no ill effect elsewhere. The architecture has strict rules about the use of pointers, in particular about using int types as if they were pointers. Breaking these rules can lead to a hardware exception at runtime.

No new tests implemented and no tests newly failing.