This adds the ability to add feature flags (e.g. `--features wasi-nn`) when compiling `wasmtime-bench-api` to allow benchmarking Wasmtime with WASI proposals included. Note that due to https://github.com/rust-lang/cargo/issues/5364, these features are only available:
- in the `crates/bench-api` directory, e.g. `pushd crates/bench-api; cargo build --features wasi-crypto`
- or from the top-level project directory using `-Zpackage-features`, e.g. `OPENVINO_INSTALL_DIR=/opt/intel/openvino cargo +nightly build -p wasmtime-bench-api -Zpackage-features --features wasi-nn`
This helps us avoid measurement bias due to accidental locality of unrelated
heap objects. See *Stabilizer: Statistically Sound Performance Evaluation* by
Curtsinger and Berger for details (although Stabilizer deals with much more than
just the location of heap allocations):
https://people.cs.umass.edu/~emery/pubs/stabilizer-asplos13.pdf
The new crate introduced here, `wasmtime-bench-api`, creates a shared library, e.g. `wasmtime_bench_api.so`, for executing Wasm benchmarks using Wasmtime. It allows us to measure several phases separately by exposing `engine_compile_module`, `engine_instantiate_module`, and `engine_execute_module`, which pass around an opaque pointer to the internally initialized state. This state is initialized and freed by `engine_create` and `engine_free`, respectively. The API also introduces a way of passing in functions to satisfy the `"bench" "start"` and `"bench" "end"` symbols that we expect Wasm benchmarks to import. The API is exposed in a C-compatible way so that we can dynamically load it (carefully) in our benchmark runner.
