Fix precedence for quantifiers and triple-op exprs

[jaybosamiya]

Fixes #25

Basically, rather than considering &&& and ||| to be prefix+binary operations (as the actual Verus parser does), we instead split them out into their own expression group. This allows us to fix the parsing precedence with respect to quantifiers.

[parno]

This looks reasonable to me, and I don't see places where it would create conflicts with block_exprs (as further evidenced by the block-related tests still passing).