Building the standard library from source

# Goals
Build libcore, liballoc, and their dependencies from source using `rules_rust`, preferably with bzlmod.
# Reasoning
When compiling to bare-metal targets, as well as custom targets, one is required to recompile the standard library from source. Currently, this is not well-supported by `rules_rust`.
# Prior art
In [bazel-rust-cross](https://github.com/colatkinson/bazel-rust-cross), @colatkinson implemented this using `rust_repositories`, although that rule is now deprecated.
In #2068, @daivinhtran made a first attempt at this, although it was quite bulky. It also manually downloaded the rust source files.