Remove functionality specified only in Annex B?

Several bits of functionality we support—notably, the functions `escape`, `unescape` and `String.prototype.substr`—appear only in [Annex B](https://www.ecma-international.org/ecma-262/5.1/index.html#sec-B) of the ES5.1 spec.

We should consider removing these since they are not required for backwards compatibility (as they would be in a web browser) and generally duplicate better functionality found elsewhere