Add preliminary support for throwing and catching uncatchable exceptions.

Additional info:
- WebAssembly "traps" are uncatchable, but are still caught by the trampoline caller.

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)
-        i32.const 42)
+        unreachable)
       (export "select_trap_l" (func 0))
     )
 "#;