Add rustc tool layer

[rene-descartes2021]

In pursuit of zero-install philosophy will download and build nightly Rust and cargo from source, if they are not on the system/PATH or the system Rust is too old.

Benefits:

• Rust/cargo can be used to satisfy plugin dependencies, using them embedded within the Vim application and not requiring administrative privileges or end-user having to read and follow installation directions for each dependency.
  • Some language server plugins require cargo to install certain language servers.
  • Use of this layer will allow for a full featured vim-clap (requires nightly Rust).
  • Can use cargo to build-from-source and embed fd/git-delta/bat/ripgrep (used by fzf/vim-clap) rather than expect end-user to administrate their install.
  • This layer can then be used to automatically build Deno and use denops.vim Vim plugins.

Similar to #491, but this builds Rust, not Node.js.

Contingent on: #500.