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
Mostly just tweaks to docs/naming/readability/tidying up.
The biggest thing is that the wasm bytes are passed in during compilation now,
rather than on initialization, which lets us remove the lifetime from our state
struct and makes wrangling unsafe conversions that much easier.
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.
