[#762] Use clearer notation for (-$)

Description

Problem: We're using '(-$)' quite a bit in the documentation (examples and doctests), however I find the pair notation somewhat confusing, especially when we're talking about instructions working on pairs themselves.

Solution: Introduce new data types 'ZippedStackRepr a b = a ::: b' and 'ZSNil = ZSNil', isomorphic to '(a, b)' and '()' respectively. 'infixr 5 :::'. Require their use to represent a zipped stack. This doesn't change the underlying Michelson representation, but disambiguates pairs and units from zipped stack representation.

See the diff for examples.

Related issue(s)

Resolves #762 (closed)

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