Add personality function.

- This is part of supporting exceptions in the future.

lib/runtime/examples/call.rs

@@ -6,7 +6,7 @@ static WAT: &'static str = r#"
     (module
       (type (;0;) (func (param i32) (result i32)))
       (func (;0;) (type 0) (param i32) (result i32)
-        unreachable)
+        i32.const 42)
       (export "select_trap_l" (func 0))
     )
 "#;