Simplify transformation visitor pattern impl

[jdpage]

Previously, we'd have AstNode.transform dynamically call the enter_ and exit_ methods. Then the pattern system would implement __getattr__ to catch all of them, which makes it hard to follow.

Now, AstNode.transform calls transform_enter and transform_exit, and the base transformer does the dynamic dispatch. The pattern system doesn't inherit from it, so no dynamic behavior there.

As a side benefit, this also simplified inheriting from ScopedPass.

[coveralls]

Pull Request Test Coverage Report for Build 203

• 36 of 40 (90.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.3%) to 84.872%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
jeff65/gold/pattern.py	11	15	73.33%
<!--	Total:	36	40	90.0%	-->

Totals
Change from base Build 193:	1.3%
Covered Lines:	1127
Relevant Lines:	1305

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💛 - Coveralls