This commit changes implementation of top-level-await in "deno_core".
Previously promise returned from module evaluation was not awaited,
leading to out-of-order execution of modules that have TLA. It's been
fixed by changing "JsRuntime::mod_evaluate" to be an async function
that resolves when the promise returned from module evaluation also
resolves. When waiting for promise resolution event loop is polled
repeatedly, until there are no more dynamic imports or pending
ops.
