Investigate crates_vendor() as an alternative to crates_repository()

[dmitrii-ubskii]

crates_vendor defines a runnable target that will generate a folder full of BAZEL.build files, similar to what cargo-raze did. Crucially, it can be executed with bazel run //.../vendor -- --repin which may be faster and/or less susceptible to the WORKSPACE issues crates_repository is. Cf. #411