ctest: Add translation of Rust types.

[mbyx]

Description

The ability to translate various Rust types that are translatable into equivalent C types. Additionally some basic unit tests have been added.

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[mbyx]

There's quite a lot of unreachable or unimplemented because the original ctest2 did the same. If that should be changed to minimize it then please tell. This is mostly a direct port of the original code over to syn, there may be some bugs.

[mbyx]

Tests seem to fail at a parse int error, so rustc --version is giving a different result there for some reason. EDIT: I forgot nightly is a thing. That's embarassing.

[mbyx]

The error on msvc goes away if I replace const bool ON = true with #define ON true in the hierarchy.h file, but I don't know why it doesn't allow it.
freebsd-15 is spurious but freebsd-13 seems to also error on hierarchy, and most likely something in generate_files is causing it.

[mbyx]

From what I understand powerpc64-unknown-linux-gnu is being cross compiled on a normal linux distro, causing a mismatch in the library file and the environment it is being built in.