* bench: add more WASI benchmarks
This follows up on #5274 to add several more scenarios with which to
benchmark WASI performance:
- `open-file.wat`: opens and closes a file
- `read-file.wat`: opens a file, reads 4K bytes from it, then closes it
- `read-dir.wat`: reads a directory's entries
Each benchmark is hand-crafted WAT to more clearly control what WASI
calls are made. As with #5274, these modules' sole entry point takes a
parameter indicating the number of iterations to run in order to use
`criterion`'s `iter_custom` feature.
* fix: reduce expected size of directory entries
In order to properly understand the impact of providing thread-safe
implmentations of WASI contexts (#5235), we need benchmarks that measure
the current performance of WASI calls using Wiggle. This change adds
several common WASI scenarios as WAT files (see `benches/wasi/*.wat`)
and benchmarks them with `criterion`. Using `criterion`'s `iter_custom`,
the WAT file runs the desired number of benchmark iterations internally
and the total duration of the runs is divided to get the average time
for each loop iteration.
Why WAT? When compiling these benchmarks from Rust to `wasm32-wasi`, the
output files are large, contain other WASI imports than the desired
ones, and overall it is difficult to tell if we are measuring what we
expect. By hand-writing the WAT, it is (slightly) more clear what each
benchmark is doing.
