feat(rust): Display toolchain names (#3414)

This is an actualization of PR #559 as originally envisioned by qryxip.

Adds the ability to display toolchain versions, either as extracted from
environment/settings files or by getting the host triple. As part of
this, several other major changes were needed:
- Many of the smaller functions within the code have been fused, moved,
  or dropped.
- The Rustup environmental info is now initialized lazily using
  OnceCells. This will hopefully lead to a performance increase.
- New configuration variables (`toolchain` and `numver`) have been added
  to allow finer-grained configuration.
- Override information is no longer read from `rustup` output. Instead,
  it is parsed from the same files that rustup would use to determine
  this info.

Co-authored-by: qryxip <qryxip@gmail.com>

Co-authored-by: qryxip <qryxip@gmail.com>

docs/config/README.md

@@ -2991,11 +2991,13 @@ The module will be shown if any of the following conditions are met:
 
 ### Variables
 
-| Variable | Example           | Description                          |
-| -------- | ----------------- | ------------------------------------ |
-| version  | `v1.43.0-nightly` | The version of `rustc`               |
-| symbol   |                   | Mirrors the value of option `symbol` |
-| style\*  |                   | Mirrors the value of option `style`  |
+| Variable  | Example           | Description                                  |
+| --------- | ----------------- | -------------------------------------------- |
+| version   | `v1.43.0-nightly` | The version of `rustc`                       |
+| numver    | `1.51.0`          | The numeric component of the `rustc` version |
+| toolchain | `beta`            | The toolchain version                        |
+| symbol    |                   | Mirrors the value of option `symbol`         |
+| style\*   |                   | Mirrors the value of option `style`          |
 
 *: This variable can only be used as a part of a style string