I elected to specify this as a `property_modifier` given that it's a property of
both actors and functions. I considered creating a whole `actor_declaration`
node but that seemed a little much. Happy to move this around if needs be.

Recognize `distributed` as a modifier on actors and functions.

This will allow components of regexen to be highlighted on GitHub, in the manner
of those in JS and Ruby.

This adds new rules for parameter packs and expansions in both type and
expression position. Since this includes the keyword "repeat", which was
already a keyword for a do-while loop, we must modify the do-while loop
rule so that it remains valid long enough to disqualify the
parameter-pack interpretation.

With this change, it's starting to look like the "contextual keyword
that is special cased as an identifier" pattern is going to become more
standard. To facilitate that, this creates an explicit (hidden) rule for
it.

Fixes #381

This reverts commit feb5398.