Refactor Rust toolchain management

- Reuse system installed rustup
- Reuse installed rustup across hooks
- Reuse installed Rust toolchains across hooks

Author: j178

docs/todo.md

@@ -15,6 +15,7 @@ The original pre-commit supports hooks written in 10+ languages. The table below
 | python ⭐ | ✅ Supported | — | `prek` supports automatic version management of Python toolchains. |
 | node | ✅ Supported | — | |
 | golang | ✅ Supported | — | |
+| rust | ✅ Supported | — | |
 | lua | ✅ Supported | — | |
 | system | ✅ Supported | — | |
 | script | ✅ Supported | — | |
@@ -24,7 +25,6 @@ The original pre-commit supports hooks written in 10+ languages. The table below
 | fail | ✅ Supported | — | |
 | deno ⭐ | 🚧 WIP | — | Experimental support in `prek`; upstream `pre-commit` lacks a native `deno` language. |
 | ruby | 🚧 WIP | [#43](https://github.com/j178/prek/issues/43) | `prek` does not currently support downloading new Ruby versions, but can use multiple simultaneously installed interpreters |
-| rust | 🚧 Planned | [#44](https://github.com/j178/prek/issues/44) | |
 | conda | 🚧 Planned | [#52](https://github.com/j178/prek/issues/52) | |
 | coursier | 🚧 Planned | [#53](https://github.com/j178/prek/issues/53) | |
 | dart | 🚧 Planned | [#51](https://github.com/j178/prek/issues/51) | |