clang::Type::num_elements should return Option<usize>

[fitzgen]

We make an FFI call to clang_getNumElements but don't check the return value. We should check for failure and return None if we find it, only casting to usize and returning Some after we've verified a successful result.

http://clang.llvm.org/doxygen/group__CINDEX__TYPES.html#gac5f636020c388126bec572cb1fb13007

If a type is passed in that is not an array or vector type, -1 is returned.

This will be one less foot gun for callers that currently need to remember to check (for std::usize::MAX == -1 as usize? I doubt any do...).

I can mentor whoever wants to work on this.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[catdesk]

I'm working on it!