typechecker+tests: Ensure all function types are known before typechecking declarations

[cg-jl]

Fixes #380 (again). PR #388 introduced a bug where it would use the function's parent scope when the function was not generic, but now it was reverted and the typechecker does a separate pass after registering functions to infer types of fat arrow functions that don't have their type stated explicitly in the source code.

[BuggieBot]

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[mustafaquraish]

Please update the tests to follow the new format from 99a337b5734ac8f8c74ecdcb545dfe9cabde956d

[cg-jl]

I reset the pull request so I can rework it from the beginning.

[cg-jl]

I've made merging this into a git nightmare. I'll start again, basing my branch off main.

[cg-jl]

Merged with module system.

[cg-jl]

Closing because Rust based compiler was discontinued, and typecheck passes are different in selfhost.