Use serde to encode/decode various TOML formats

[djc]

This saves a bunch of code (-190 lines) and is generally more idiomatic. It also saves about 8% of latency on proxy invocations, after (both best of 3 runs):

djc-2021 toml rustup $ RUSTUP_FORCE_ARG0=rustc hyperfine "./target/release/rustup-init --version"
Benchmark 1: ./target/release/rustup-init --version
  Time (mean ± σ):      32.6 ms ±   0.6 ms    [User: 23.3 ms, System: 5.5 ms]
  Range (min … max):    31.4 ms …  34.6 ms    86 runs

before:

djc-2021 master rustup $ RUSTUP_FORCE_ARG0=rustc hyperfine "./target/release/rustup-init --version"
Benchmark 1: ./target/release/rustup-init --version
  Time (mean ± σ):      35.3 ms ±   0.6 ms    [User: 25.6 ms, System: 5.8 ms]
  Range (min … max):    33.6 ms …  36.6 ms    79 runs

[rami3l]

To me it's clear that the happy path in the original code is for MetadataVersion::default(), but now it's for MetadataVersion::V12. I know at the moment being it's all the same, but the meaning being conveyed seems to be a bit different, and that might cause problems when the default value changes 🤔

[djc]

I've reverted the logic to something closer to what was originally there.

[rami3l]

Nice improvements overall! Somehow I thought we were using serde for such things already 🤣