Sequences Are Cool™️

• Fully rewrite Sequences to use generators a0af35d
• Change methodMissing to _methodMissing 7fb980b
• Further clean up types and move tests around 8841fa0
• Fix types for generate, before, after; added tests c2b181a
• Change/clean up sequence type signatures, make sequences global 660e243
• Add complicated association test with join tables dd7b34e
• Cleaned up a lot of internal types, allow for Custom Strategy classes to be passed in f0c014d

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