Rework hl interpolator

[allanrenucci]

We are currently having issues with the hl interpolator.

After adding syntax highlighting to Showable, we now sometime leaks colors in error messages (e.g. #3614). The reason is because we sometime try to syntax highlight twice (first by calling show and then splicing the result within the hl interpolator).

I propose we change the API of the hl interpolator as follow:

def hl(args: Showable*)(implicit ctx: Context): String

A user of the interpolator will now need to be explicit when splicing Strings with the hl interpolator. It is a bit cumbersome, but will prevent re-coloring. We can provide helper function (or interpolator) to go from String to Showable.

@felixmulder WDYT?

[felixmulder]

Sounds pretty good, looking forward to seeing where this will lead.

One thing that has been missing is some form of best-effort line splitting. We have the width as a compiler setting, so we should be able to reflow text. If this covers that - all the better :)

[smarter]

I think we should go further and get rid of thehl interpolator completely, just use em/ex which should be calling .show which should do the coloring when needed, and we can always have a regular hl method when we want to do highlighting explicitly, e.g.

em"No explicit ${hl("return")} allowed here."