Adding this.context and Thread.backtrace()

Adding this.context and Thread.backtrace()
Unused arguments should be silenced just after the variable declarations, but before the first statement. For example:

```C
static void
gum_script_invocation_context_on_get_context (Local<String> property,
    const PropertyCallbackInfo<Value> & info)
{
  GumFoo * foo = ...; /* In GumScript, which is C++ because of V8, only */
                      /* common variables are declared at the top.      */
                      /* Note that in the rest of Gum, which is C,      */
                      /* variable declarations are always at the top in */
                      /* order to be compatible with Microsoft's C      */
                      /* compiler.                                      */

  (void) unused_argument;

  first_statement;
  second_statement;

  return statement;
}
```