Add CPlusPlus support

[mattip]

The codebase did not support compiling with a c++ compiler. I also added a CI run for g++

[steve-s]

If it works I am all for this. Most of the changes look sane even for pure C codebase. But doesn't testing everything with g++ push it farther than necessary? We "only" need the header files C++ compatible (*), i.e., the static inline functions and macros, which get pulled into the C++ code of the C++ extension, but other things that just link with the extension are fine to be C only?

(*) plus the C files that are currently compiled with the extension, but as we discussed in the meeting, we'd like to actually compile them ourselves, so in that case they'd be always compiled by C compiler too).

[mattip]

Closes #318. In order to do this the PR:

• Adds a CI run using g++ for tests on ubuntu
• Allows overriding gcc in the test/Makefile
• Moves HPyTuple_Pack from a macro to an function in inline_helpers.h
• Adds guards when exporting functions to prevent mangling the names
• Casts malloc (and friends) results to the proper pointer type
• Fixes tests to run with C++
  • Casts where needed
  • Move code so variable declarations are not between a goto xxx and the xxx label
  • Reorder HPyType_Spec initializations so that the order matches the struct definition

The new CI run is passing.

[mattip]

Would anyone else like to review or can this be merged?

[qunaibit]

LGTM

[mattip]

ping

[mattip]

This PR has two approvals, can we merge it?

[mattip]

🚀