D guest bindings generator

[QuantumSegfault]

Implements a d subcrate to support generating bindings for the D programming language.

[QuantumSegfault]

This PR is not yet finished. Just opening a draft early to help interested parties keep track.

[alexcrichton]

Thanks! One thing I'd also recommend as you're implementing things is to add D to this enum as well to hook this into the testing infrastructure. You can then add d to this matrix as appropriate with CI-specific steps to setup the compiler and environment. That'll get codegen/runtime tests working and you'll be able to test out your work by dropping files into the tests/runtime/* folder to implement various components. You'll need to write some orchestration for the test runner to know how to compile D programs (you can draw inspiration from how C is organized) too.

Happy to help answer questions about anything in specific if you have them, and if you have any questions about Rust/idioms/etc feel free to leave a comment here and I can dig in. Otherwise I'll leave this be until you're ready, in which case feel free to ping me and I can take a closer look.