Implement Symbol type when the Symbol global is available.

[papandreou]

Fixes #261.

Symbols can also be used as keys in objects -- that's not implemented yet.

[sunesimonsen]

Cool but CI fails.

[sunesimonsen]

It is just some listing errors.

[papandreou]

Yeah, seems like our jshint does not know that Symbol should be an exception to the "all identifiers starting with a capital letter are constructors and must invoked with new" rule. Maybe we can fix it by upgrading, but we might have to specify esversion: 6 for it to work. I'll look into it.

[sunesimonsen]

Linting is just never fun, tried to upgrade standard today and got 100+ errors :-S

[papandreou]

I think I sorted it out now.

Also added support for Symbols as keys in objects. Even though I'm not totally satisfied with how that turned out, I think we'll need to include it at some point for completeness. Let me know what you think.

[sunesimonsen]

Very nice 👍

[papandreou]

Raising the coverage further is tricky. The keyComparator doesn't get fully exercised because getKeys returns the keys followed by the symbols.