Introduce benchmarking API

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.

crates/bench-api/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "wasmtime-bench-api"
+version = "0.19.0"
+authors = ["The Wasmtime Project Developers"]
+description = "Exposes a benchmarking API for the Wasmtime runtime"
+license = "Apache-2.0 WITH LLVM-exception"
+repository = "https://github.com/bytecodealliance/wasmtime"
+readme = "README.md"
+edition = "2018"
+publish = false
+
+[lib]
+name = "wasmtime_bench_api"
+crate-type = ["rlib", "cdylib"]
+# The rlib is only included here so that `cargo test` will run.
+
+[dependencies]
+anyhow = "1.0"
+wasmtime = { path = "../wasmtime", default-features = false }
+wasmtime-wasi = { path = "../wasi" }
+wasi-common = { path = "../wasi-common" }
+
+
+[dev-dependencies]
+wat = "1.0"