Misc refactoring, imports, directive "requires", and validation

Add a few language features:

• Imports - includes types and directives from a resolver function. In practice, this will load from ~/.wapc/definitions which is installed to wapc install (upcoming PR in the CLI)
• Directive requires @other on TYPE | etc. syntax to target what other directives must exist in relation to it (validation forthcoming)

Refactored:

• Added validate function that takes in configuration rules (which as visitors in rules)
• Kinds are now an enum - no more instanceof
• Organization of classes into definitions, nodes, types & values
• More visitor hooks