Generated name override

[justsmth]

Addresses issue: #1375

• Adds generated_name_override callback to the ParseCallbacks trait.
• Callback is invoked for each function name (after other name-related logic).
• Added test to verify.

[bors-servo]

☔ The latest upstream changes (presumably 566d226) made this pull request unmergeable. Please resolve the merge conflicts.

[justsmth]

The msrv check above is failing with the following message:

Run rm Cargo.lock && cargo +1.54.0 build --lib
    Updating crates.io index
 Downloading crates ...
  Downloaded clap_lex v0.2.2
error: failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc[6](https://github.com/rust-lang/rust-bindgen/runs/6907225276?check_suite_focus=true#step:4:7)299db9ec[8](https://github.com/rust-lang/rust-bindgen/runs/6907225276?check_suite_focus=true#step:4:9)23/clap_lex-0.2.2/Cargo.toml`
Caused by:
  feature `edition2021` is required
  this Cargo does not support nightly features, but if you
  switch to nightly channel you can add
  `cargo-features = ["edition2021"]` to enable this feature
Error: Process completed with exit code [10](https://github.com/rust-lang/rust-bindgen/runs/6907225276?check_suite_focus=true#step:4:11)1.

Is this a problem related to my change?

[justsmth]

I created a separate PR for the bump to 1.56.1: #2225

[justsmth]

Moving to cleaned up PR: #2228