Release 0.20.0

[kkysen]

• Fixes Latest version on crates.io is 0.18.0 instead of 0.19.0 #1125.
• Fixes Cannot install with "cargo install c2rust" #1199.

Following as much from c2rust/wiki/Release-Process as still works/applies, plus some extra additions, I ...

• cargo outdated -R: doesn't work due to unknown print request split-debuginfo
• cargo update: doesn't work because of old nightly
• cargo clippy:
  • cargo clippy --fix
  • Other trivial cargo clippy fixes
• cargo fmt --check
• Updated versions from 0.19.0 to 0.20.0.
• Specified missing versions for relative dependencies.
• cargo build
• cargo +stable build -p c2rust
• Set publish = false in crates that are unstable due to #![feature(rustc_private)].

Then I will publish the crates in this order with cargo +stable -p $package:

• c2rust-asm-casts
• c2rust-ast-builder
• c2rust-ast-printer
• c2rust-build-paths
• c2rust-ast-exporter
• c2rust-bitfields-derive
• c2rust-bitfields
• c2rust-transpile
• c2rust

or if this works, then I'll use this:

cargo +nightly publish --workspace --dry-run -Z package-workspace --exclude c2rust-analysis-rt --exclude c2rust-instrument --exclude c2rust-pdg --exclude c2rust-analyze

I need to use cargo +stable for this because the pinned nightly does not correctly resolve the Cargo.lock for a workspace, and thus tries to incompatibly update it.

[fw-immunant]

LGTM. 🚀