Add support for let, const and for..of loops

[fiddlerwoaroof]

Adds support for some low-hanging fruit from the ES6 spec

[marijnh]

I think if we're going to add support for new syntax, that should happen behind an option, say :ecma-version, because otherwise it is likely to break existing uses of the library.

I'm not maintaining this anymore, so if you want, I can give you access and you can work on it, but keep in mind that quicklisp is directly pulling from this repository, so if you make compatibility-breaking changes you're likely to cause problems for people using it.

Also, the ES6 (and up) standard is a complex beast, and requires some studying before you approach it -- let, for example, isn't a regular keyword, but a contextual one, so that any previously valid usages of the identifier let continue to work (try var let = 10 in your browser console, for example).