Split off definition of AST into a separate crate

[TomAFrench]

noirc_evaluator currently depends on noirc_frontend because it makes usage of the AST which is defined in noirc_frontend. It would then help speed up our compilation times if we can split the AST definition into a separate noirc_ast crate which noirc_frontend and noirc_evaluator can depend on.

Beyond the compilation times, this would also allow us to split off some more crates from noirc_frontend for organisation such as the parser.

For reference, Rust does the same and explicitly makes no stability guarantees about this crate: https://doc.rust-lang.org/stable/nightly-rustc/rustc_ast/index.html

cc @jfecher as I imagine you made a good deal of the decisions on the current structure plus you've expressed views on this in the past.

[jfecher]

Talked about this at standup: I'm alright with splitting off the monomorphized AST to see if it'll improve compile-times. No longer needing to recompile the evaluator after the frontend is changed would be nice. If we have more dependencies on data structures cross crates in the future (e.g. the initial AST and the formatter) we should consider breaking those up as well.