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>

Cargo.toml

@@ -83,6 +83,8 @@ yaml-rust = "0.4.5"
 
 process_control = { version = "3.4.0", features = ["crossbeam-channel"] }
 
+guess_host_triple = "0.1.3"
+home = "0.5.3"
 shell-words = "1.1.0"
 
 [dependencies.schemars]