feat: use singleton WasmBlackBoxFunctionSolver in noir_js (#3966)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

This PR stores a `WasmBlackboxFunctionSolver` in a global variable so
that we can avoid reinitialisation on subsequent exections.

This takes a trick from `aztec-packages` but I haven't validated that it
actually causes a speedup

## Additional Context


## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** 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.

tooling/noir_js/src/witness_generation.ts

@@ -1,8 +1,24 @@
 import { abiEncode, InputMap } from '@noir-lang/noirc_abi';
 import { base64Decode } from './base64_decode.js';
-import { executeCircuit, WitnessMap, ForeignCallHandler, ForeignCallInput } from '@noir-lang/acvm_js';
+import {
+  WitnessMap,
+  ForeignCallHandler,
+  ForeignCallInput,
+  createBlackBoxSolver,
+  WasmBlackBoxFunctionSolver,
+  executeCircuitWithBlackBoxSolver,
+} from '@noir-lang/acvm_js';
 import { CompiledCircuit } from '@noir-lang/types';
 
+let solver: Promise<WasmBlackBoxFunctionSolver>;
+
+const getSolver = (): Promise<WasmBlackBoxFunctionSolver> => {
+  if (!solver) {
+    solver = createBlackBoxSolver();
+  }
+  return solver;
+};
+
 const defaultForeignCallHandler: ForeignCallHandler = async (name: string, args: ForeignCallInput[]) => {
   if (name == 'print') {
     // By default we do not print anything for `print` foreign calls due to a need for formatting,
@@ -26,7 +42,12 @@ export async function generateWitness(
   // Execute the circuit to generate the rest of the witnesses and serialize
   // them into a Uint8Array.
   try {
-    const solvedWitness = await executeCircuit(base64Decode(compiledProgram.bytecode), witnessMap, foreignCallHandler);
+    const solvedWitness = await executeCircuitWithBlackBoxSolver(
+      await getSolver(),
+      base64Decode(compiledProgram.bytecode),
+      witnessMap,
+      foreignCallHandler,
+    );
     return solvedWitness;
   } catch (err) {
     throw new Error(`Circuit execution failed: ${err}`);