Better compression by using ES6 features

[ThomasR]

All currently supported browsers are more or less ES6 compatible. So:

• Instead of function, use =>
• ~~Replace .map.call(foo, function(bar){…}); by `for (let bar of foo) {…}``~~ see Chromium bug
• Use template string (in case of 'a'+b+'c' -> a${b}c, not in case of 'a'+b or b+'c', since that yields no advantage)

Unfortunately, neither uglifyjs nor esprima/esmangle/escodegen support this. There are four options:

• Wait for estools / uglifyjs to implement that stuff
• Fork and implement the required features
• Use a totally different tool (Some transpiler? Babel Plugin? Traceur?)
• Manually edit the minified result