Disable cranelift's verifier by default (#882)

The intention of the `wasmtime` crate was to disable this verifier by
default, but it looks like cranelift actually has it turned on by
default which was making our documentation incorrect!

This was discovered by seeing a number of timeouts when fuzzing. The
debug verifier is great for fuzzing, however, so fuzzing is updated to
enable this unconditionally, meaning we'll still have timeouts. For
general users though this should make the documentation correct that the
`wasmtime` crate, by default, disables the debug verifier.

crates/api/src/runtime.rs

@@ -35,6 +35,11 @@ impl Config {
             .enable("avoid_div_traps")
             .expect("should be valid flag");
 
+        // Invert cranelift's default-on verification to instead default off.
+        flags
+            .set("enable_verifier", "false")
+            .expect("should be valid flag");
+
         Config {
             debug_info: false,
             features: Default::default(),