Implement the unique-id() function

This PR implements the `unique-id` function introduced in Ruby sass 3.3 by returning a random unique CSS identifier. The implementation is pretty much an exact copy of the Ruby sass implementation https://github.com/sass/sass/commit/f3be0f40b7c5072635b2d6dab772f2c29e8d629a.

This implementation is slightly different to the Ruby implement which

> Returns a unique CSS identifier within the scope of a single CSS file.

I'm not sure the scoping serves any particular benefit but maybe @chriseppstein knows better?

Fixes https://github.com/sass/libsass/issues/636. Not sure how best to spec this to be honest.
The hex pipe isn't technically required, and is probably an unnecessary performance hit. The intent was to function more like the Ruby implementation.

We can get away prepend the random number with a string ( i.e. "u" ) to create a valid identifier.