Add a test for nested unquotes

[masak]

Just realized in #567 that unquote (the alternative {{{ }}} syntax) supports nested unquotes, and that we don't have test coverage for that at all. We should add at least one test for that.

Yes, that will mean implementing the unquote syntax in the first place, one way or another. It's high time we did that.

[masak]

As part of writing such a test, we also need to decide what the actual semantics of nested unquoting should be. I think the two main candidates are:

• Interpolate N levels out (as in Lisp quasiquotes)
• Disallow it with a compile-time error

And I don't think this behavior is a contender:

• Just pretend we didn't see the inner quote, and interpolate 1 level out (either failing, or succeeding in some unlikely way that might as well have been a failure)