[Kenny Kerr (kennykerr)]

This update introduces a low-level library for reading and writing the ECMA-335 format used by Windows to describe APIs.

I have written numerous parsers for ECMA-335 over the years but have struggled to come up with a decent generator, but I think I have finally landed on a capable low-level library design and implementation for both reading and writing metadata efficiently and concisely. As an example, I used it to write an MDMERGE knockoff in about 100 LOC.

You might say this is a reversal of #3266 where I made the decision to focus on windows-bindgen, but I think that Rust binding generation is now sufficiently mature that I can once again revisit metadata generation, as the weakest link in the toolchain. So this is the first step in that direction.

This update includes:

• windows-ecma335 library crate with the low-level reader and writer.
• test_ecma335 crate with basic test coverage for this library.
• tool_merge as a basic MDMERGE-like tool for merging .winmd files.
• Various unrelated Clippy nightly suggestions to pass build validation

Next steps:

• Improve test coverage.
• Update windows-bindgen to use this metadata reader so we only have one Rust parser.
• Build front-ends to read IDL and header files to produce metadata directly using the windows-ecma335 library.