Add bazel rules for existing rust code

[cfrantz]

Addresses: #105

[cfrantz]

This is still a WIP, but soliciting feedback. This is a naive conversion of the build system to bazel for the existing code in the repo.

This commit converts all of the existing rust crates over to rules_rust except the xtask and openprot crates.

• I've named all of the rules the same name as their bazel package (e.g. sort-of "subdir"). This means that bazel labels like //hal/blocking are equivalent to //hal/blocking:blocking.
• I've used the crate_name attribute to give the crates the same crate name as the name in their Cargo.toml.
• I've put all upstream crate deps into //third_party/crates_io/Cargo.toml, which is processed by bazel rules_rust (see MODULE.bazel to see how that happens).

Not yet done:

• I haven't yet removed the various Cargo.toml files. We have a couple of options:
  • Remove the Cargo.toml files and become a pure bazel repo.
  • Try to keep the Cargo.toml files and be a bazel+cargo repo. I consider this unlikely to work once we start integrating with the pigweed kernel or code from other languages (e.g. C/C++ code). Pigweed is not cargo compatible. Integrating rust with C/C++ code for both bazel and cargo will quickly turn into an exercise in frustration.

The build system conversion in this PR does not address #95, #97 or #98.