wiggle: introduce wiggle::Trap, which can be either a String or I32

also, make noreturn functions always return a Trap

wasmtime-wiggle can trivially turn a wiggle::Trap into a wasmtime::Trap.
lucet will have to do the same.

crates/wiggle/src/lib.rs

@@ -928,3 +928,9 @@ impl Pointee for str {
         <[u8]>::debug(pointer, f)
     }
 }
+
+#[derive(Debug, Clone)]
+pub enum Trap {
+    I32(i32),
+    String(String),
+}