Implement simple benchmarks for instantiation.

This adds benchmarks around module instantiation using criterion.

Both the default (i.e. on-demand) and pooling allocators are tested
sequentially and in parallel using a thread pool.

Instantiation is tested with an empty module, a module with a single page
linear memory, a larger linear memory with a data initializer, and a "hello
world" Rust WASI program.

benches/.gitignore

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+instantiation/wasi.wasm