Add `rust_sitrep()` function

[Ilia-Kosenkov]

Closes #168

Add rust_sitrep() function to report on the state of rust infrastructure to the best of our ability. Rely on cargo and rustup to determine versions of the build system, host architecture, available targets, and default toolchain. If anything is missing, suggest using rustup. rust_sitrep() does not make an attempt to gues if the appropriate target is installed.

rextendr::rust_sitrep()
#> Rust infrastructure sitrep:
#> ✔ "rustup": 1.25.1 (bb60b1e89 2022-07-12)
#> ✔ "cargo": 1.68.2 (6feb7c9cf 2023-03-26)
#> ℹ host: x86_64-pc-windows-msvc
#> ℹ toolchain: stable-x86_64-pc-windows-msvc
#> ℹ targets: i686-pc-windows-gnu, x86_64-pc-windows-gnu, and
#>   x86_64-pc-windows-msvc

^{Created on 2023-05-13 with reprex v2.0.2}

Here we pretend nothing is installed

rextendr::rust_sitrep()
#> Rust infrastructure sitrep:
#> ✖ "rustup": not found
#> ✖ "cargo": not found
#> ✖ Cannot determine host, toolchain, and targets without "rustup"
#> ✖ "cargo" is required to build rextendr-powered packages
#> ℹ It is recommended to use "rustup" to manage "cargo" and "rustc"
#> ℹ See <https://rustup.rs/> for installation instructions

^{Created on 2023-05-13 with reprex v2.0.2}

[CGMossa]

Omg so far this thing looks clean.. 👍