perf(quickcheck): execute zipped generators directly

[mizchi]

Implement Generator::zip_with and zip_with3 directly instead of composing flat_map and map. The previous composition allocated temporary generators and closures for every generated sample.

The state splitting and evaluation order are preserved.

Native benchmark (zip_with, 1,000,000 samples):

before	after	change
38.66 ms	11.64 ms	70% faster

Validation: all 82 QuickCheck tests pass on wasm, wasm-gc, JS, and native; moon check --target all and moon info pass.

[Copilot]

Pull request overview

This PR optimizes @quickcheck.Generator composition by implementing Generator::zip_with and Generator::zip_with3 directly, avoiding per-sample allocation overhead from flat_map/map composition while preserving the same RandomState::split() sequencing semantics.

Changes:

• Implement Generator::zip_with via a single generator closure that splits state once and runs both generators directly.
• Implement Generator::zip_with3 directly, matching the previous nested-flat_map state-splitting behavior.
• Add a benchmark-style test for Generator::zip_with and wire in the bench dependency for tests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
quickcheck/moon.pkg	Adds moonbitlang/core/bench as a test-only import to support benchmark tests.
quickcheck/generator.mbt	Replaces composed flat_map/map implementations of zip_with/zip_with3 with direct implementations to reduce allocation/closure overhead.
quickcheck/generator_bench_test.mbt	Adds a benchmark test exercising Generator::zip_with for performance tracking.

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