Document and static assert assumptions on size_t layout

[dtolnay]

Rust specifies that usize is ABI compatible with C's uintptr_t: https://rust-lang.github.io/unsafe-code-guidelines/layout/scalars.html#isize-and-usize. However there is no direct Rust equivalent for size_t. C does not guarantee that size_t and uintptr_t are compatible. In practice though, on all platforms supported by Rust, they are identical for ABI purposes. See the libc crate which unconditionally defines libc::size_t = usize. We expect the same here and the assertions in this PR are just to explicitly document that.

Note that no assumption is made about C++ name mangling of signatures containing these types, not here nor anywhere in CXX.

[dtolnay]

Some background on this topic:

• size_t vs usize rust-lang/rust-bindgen#1671
• size_t_is_usize should default to true rust-lang/rust-bindgen#1901
• Is it possible to map usize to size_t and isize to ptrdiff_t? mozilla/cbindgen#239