Refactor of the frontend

[W95Psp]

This PR is a "big" refactor, I guess that's my way of killing time on this scary election Sunday in France...

Sorry for the bulky monolithic PR, that's a big refactor thing with a lot of very related changes, that would have been 5 or 10 PRs, that would have been a pain to push and merge.

• Architecture:
  • cargo-hax now:
    1. calls cargo build, which calls multiple times our custom rustc driver. Instead of running the engine itself or spitting out JSON, the rustc driver now creates a haxmeta file per crate in the target directory (along with rmeta rustc files).
       A haxmeta file is a full CBOR bincode export of the AST of a crate by hax's exporter. It contains all the data we need.
       It also communicates on stderr what haxmeta files have been produced.
       This haxmeta file mechanism respects much more what rustc does itself, allowing us to benefit from the cache of cargo and rustc. Also, we need that mechanism for things like allow trait param with empty predicates #371: with haxmeta files, we will be able to do cross-crate reasoning.
    2. reading the stderr of cargo build, we now what haxmeta files were produced.
    3. we process those haxmeta files, and call the engine
  • The engine is now communicating with cargo-hax with messages (on stdin/stdout). Before, the communication was done with one JSON input payload at the beginning and one JSON payload at the end. Now, the engine can communicate interactively with cargo-hax. This is useful for:
    • dynamic error reporting
    • rustfmt requests
    • diagnostics pretty printing
    • in the future, a nicer debugging tool
  • I removed the crates hax-cli-options, hax-cli-options-engine, and hax-diagnostics.
    • This stratification stems from us thinking we needed some crates to be nightly and some to be stable, that's not the case
    • I merged everything into a hax-types crate, which contains all the types required for communicating between cargo-hax, hax-engine and the custom rustc driver.
• Minor changes:
  • use prettyplease instead of rustfmt to format our internal Rust-ish AST (that's much faster)
  • re-enable cargo cache by default
  • errors are displayed with annotate_snippets, which is Rustc's crate for reporting errors. Closes Move diagnostics reporting to annotate_snippets #739.
  • simplify diagnostics
  • remove the linter (closes Remove linter #611)
  • clean up some type in the CLI options
  • remove pretty_print_diagnostics
  • use a different target directory to avoid recompilation
  • subsumes refactor: feature gate all implementations with full #713: adds a rustc feature to hax-frontend-exporter, making the dependency to librustc_driver optional

[franziskuskiefer]

Thanks, only a few nits.

[W95Psp]

Thanks for the review 😃 I addressed all your points I think :)

[franziskuskiefer]

Thanks, lgtm with a few more nits. Approving already, but please address them before mergin.g
Let's try to keep dependencies to a minimum and really only depend on what we need.