Load Rust rules from @rules_rust//rust:defs.bzl.

[PiotrSikora]

Following bazelbuild/rules_rust#592, this is now the preferred way to load public rules.

Unfortunately, it's not a simple replacement. The "old" rust_library from @rules_rust//rust:rust.bzl was split into multiple targets in @rules_rust//rust:defs.bzl (rust_library for lib, rlib, dylib, rust_shared_library for cdylib, rust_static_library for staticlib, and rust_proc_macro for proc-macro), and rust_library has been repurposed, so we need to generate different targets for different crate types.

cc @UebelAndre

[PiotrSikora]

This was fixed in #452.