chore: Optimize the elaborator (#5230)

# Description

## Problem\*

Resolves #5194

## Summary\*

Fixes a performance issue where the elaborator was ~2x as slow running
`nargo t --use-elaborator` on aztec-nr versus the same tests without the
elaborator (4s versus 2s).

Found via flamegraph - `self.type_variables` wasn't cleared after each
function was elaborated. This Vec is meant to just store type variables
local to a function so that we can check if any need type hints after it
is done but since it wasn't cleared out, we'd be checking an
increasingly large Vec for each function.

## Additional Context

With this change the elaborator is slightly faster (but not noticeably)
than our current code. 1.9 versus 2.0s average across 10 runs for `nargo
t` (with lower std. deviation than our current code as well).

## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_frontend/src/elaborator/mod.rs

@@ -402,6 +402,7 @@ impl<'context> Elaborator<'context> {
         meta.function_body = FunctionBody::Resolved;
 
         self.trait_bounds.clear();
+        self.type_variables.clear();
         self.in_unconstrained_fn = false;
         self.interner.update_fn(id, hir_func);
         self.in_contract = was_in_contract;