bench: add more WASI benchmarks (#5309)

* 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

benches/wasi/read-dir.wat

@@ -0,0 +1,41 @@
+;; Read the directory entries of the preopened directory.
+(module
+    (import "wasi_snapshot_preview1" "fd_readdir"
+        (func $__wasi_fd_readdir (param i32 i32 i32 i64 i32) (result i32)))
+    (func (export "run") (param $iters i64) (result i64)
+        (local $i i64)
+        (local.set $i (i64.const 0))
+
+        (if (i32.ne (i32.load (i32.const 0)) (i32.const 0))
+            (then unreachable))
+
+        (loop $cont
+            ;; Read the file into the sole iovec buffer.
+            (call $__wasi_fd_readdir
+                ;; The fd of the preopened directory; the first three are the
+                ;; `std*` ones.
+                (i32.const 3)
+                ;; The buffer address at which to store the entries and the
+                ;; length of the buffer.
+                (i32.const 16)
+                (i32.const 4096)
+                ;; The location at which to start reading entries in the
+                ;; directory; here we start at the first entry.
+                (i64.const 0)
+                ;; The address at which to store the number of bytes read.
+                (i32.const 8))
+            (drop)
+
+            ;; Check that we indeed read at least 380 bytes of directory
+            ;; entries.
+            (if (i32.lt_u (i32.load (i32.const 8)) (i32.const 300))
+               (then unreachable))
+
+            ;; Continue looping until $i reaches $iters.
+            (local.set $i (i64.add (local.get $i) (i64.const 1)))
+            (br_if $cont (i64.lt_u (local.get $i) (local.get $iters)))
+        )
+        (local.get $i)
+    )
+    (memory (export "memory") 1)
+)