implement fuzzing for component types (#4537)

This addresses #4307.

For the static API we generate 100 arbitrary test cases at build time, each of
which includes 0-5 parameter types, a result type, and a WAT fragment containing
an imported function and an exported function.  The exported function calls the
imported function, which is implemented by the host.  At runtime, the fuzz test
selects a test case at random and feeds it zero or more sets of arbitrary
parameters and results, checking that values which flow host-to-guest and
guest-to-host make the transition unchanged.

The fuzz test for the dynamic API follows a similar pattern, the only difference
being that test cases are generated at runtime.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

crates/component-util/src/lib.rs

@@ -77,3 +77,62 @@ impl FlagsSize {
 fn ceiling_divide(n: usize, d: usize) -> usize {
     (n + d - 1) / d
 }
+
+/// A simple bump allocator which can be used with modules
+pub const REALLOC_AND_FREE: &str = r#"
+    (global $last (mut i32) (i32.const 8))
+    (func $realloc (export "realloc")
+        (param $old_ptr i32)
+        (param $old_size i32)
+        (param $align i32)
+        (param $new_size i32)
+        (result i32)
+
+        ;; Test if the old pointer is non-null
+        local.get $old_ptr
+        if
+            ;; If the old size is bigger than the new size then
+            ;; this is a shrink and transparently allow it
+            local.get $old_size
+            local.get $new_size
+            i32.gt_u
+            if
+                local.get $old_ptr
+                return
+            end
+
+            ;; ... otherwise this is unimplemented
+            unreachable
+        end
+
+        ;; align up `$last`
+        (global.set $last
+            (i32.and
+                (i32.add
+                    (global.get $last)
+                    (i32.add
+                        (local.get $align)
+                        (i32.const -1)))
+                (i32.xor
+                    (i32.add
+                        (local.get $align)
+                        (i32.const -1))
+                    (i32.const -1))))
+
+        ;; save the current value of `$last` as the return value
+        global.get $last
+
+        ;; ensure anything necessary is set to valid data by spraying a bit
+        ;; pattern that is invalid
+        global.get $last
+        i32.const 0xde
+        local.get $new_size
+        memory.fill
+
+        ;; bump our pointer
+        (global.set $last
+            (i32.add
+                (global.get $last)
+                (local.get $new_size)))
+    )
+"#;