Give up and introduce circumfix and postcircumfix operators

[masak]

This will probably be necessary in #430, but it's been a long time coming.

• We've talked many times about having a Qtype for expression parentheses. It's not necessary for "regular" uses of expressions — since the Qtree layout captures all of that information anyway — but it shows up in basically any non-standard treatment of parentheses. This new construct could be a term, but it feels much closer to the truth to make it a circumfix.

• Array and dict literals could also be a circumfix, with their own special expression-like environment (Allow operators to be defined in other, "expression-like" rules #430).

• Call syntax and indexing syntax could be postcircumfixes, again with their own special expression-like environment.

Basically it feels like this change would pull a few things out of is parsed handling and into the declarative. As an extra bonus, we end up closer to Perl 6's model.