refactor kind logic, add enum, tuple, dict, union, intersection

[ianstormtaylor]

Refactors the kind logic to be a bit cleaner and more composed. This also adds dict, enum, tuple, union, intersection struct keywords as well. For example:

// tuples
const Move = struct(['string', 'number'])

// enums
const Name = struct.enum(['john', 'jane', 'jack', 'jill'])

Closes #5
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