Simplify benchmarking wrappers

[RReverser]

Description of Changes

• Reuse same runtime between module and db benchmarks.
• Use Criterion's to_async for async benchmarks.
• Reorder fields of SpacetimeModule to avoid the Option<ModuleHandle> hack - Rust has a guaranteed drop order that we can rely on.
• Avoid cloning and batching of argument product values, use references instead.

This both simplifies benchmarking code and reduces the timing overhead a bit.

API and ABI breaking changes

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Expected complexity level and risk

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Testing

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[mamcx]

LGTM