feat: Syntax for environment types now works with generics

[alexvitkov]

Description

Problem*

The syntax for closure environment types that I added in #2357 is sub-optimal as it does not play well with generics.

Resolves: #2334

Summary*

• As suggested in feat: add syntax for specifying function type environments #2357 (comment) this PR changes it changed from fn[envarg1, envarg2](arg1, arg2, ...) -> ret to fn[env_type](arg1, arg2, ...) -> ret so that a named generic can be used an environment.

• Additional error checking is also added, since the new syntax allows passing any type as an environment, but the only valid ones are Unit/NamedGeneric/Tuple.

• Added tests for generic closure environments

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

Looks good but if we move the error check to the parser we can get it a bit cleaner

[jfecher]

After testing out this PR, I think it is too confusing to report the error in a different source location. Can you add source locations to UnresolvedTypes so that the error location is correct?

[alexvitkov]

I've added spans to UnresolvedType, the error now looks like this:

error: asdkj is not a valid closure environment type
  ┌─ /home/vitkov/code/repos/metacraft-labs/noir/proj/src/main.nr:4:16
  │
4 │ fn bad(foo: fn[asdkj]() -> Field) {
  │                ----- Closure environment must be a tuple or unit type

I thought about submitting a new PR, but it turned out not to be that massive of a change, so I dropped it in here so we can get it done faster

[jfecher]

👍