Support for ECMAScript 6 and 7

[erikd]

This implements features for es 6 & 7.

• const and let
• arrow expressions
• import
• export
• default arguments
• object matching (destructuring, single name bind)
• spread operator
• classes + extends
• template strings

Each of these should be implemented in a separate PR and each item needs tests. There are likely to be three tests needed, one for the appropriate XParser.hs test and one in each if Minify.hs and RoundTrip.hs.

[erikd]

const was already supported and let as added in PR #72.

[erikd]

Support for the spread operator was added in #73.

[erikd]

Arrow expressions were added in #74.

[Cmdv]

@erikd would it be ok to work on import and export individually?
I spotted the work was done in the original big PR. Just don't want to do it if you are already working on it 😄

[erikd]

Absolutely fine.

Unfortunately, I have not done much more than merge patches for some time.

[hdgarrood]

I'd like to pick up where @Cmdv left off on import parsing.