Encapsulate rust toolchain details behind a depset with all inputs

[hlopko]

Right now, rust_toolchain has many simple fields that return rustc binary, rustc_lib files, C++ toolchain files, rust stdlib files, or with #836 also target json file.

AFAIK there is not a good reason why we need these fine grained artifacts. It will be more convenient to use and more backward-compatibility-friendly to expose one "all inputs relevant to compilations" depset that contains all users need. What's even better is that Bazel can save a bit of memory by reusing this single depset instead of creating slightly duplicated ones.

[hlopko]

Oh one more detail - the way inputs for rustc action, rustdoc action, and cargo build script action are computed is slightly different in each of them, very likely unintentionally. Another reason to provide a better API.