Destructuring

[robrix]

Implements destructuring pattern matching.

id firstIfConcatenation = [HMRCase match:combinator withCases:@[
    [HMRCase case:HMRConcatenationPredicate(HMRBind, HMREmptyPredicate) then:^(id<HMRCombinator> first) {
        return first;
    }],
]]

• [x] Try out HMRConcatenationCase, returning a HMRCase instance?
• [x] Repetitions
• [x] Reductions
• [x] Literals
• [x] Character sets
• [x] Null parses
• [x] Empty
• [x] Use destructuring to implement cyclic test
• [x] Use destructuring to implement nullability test

Fixes #15.