Move structuralize pass to separate file

[jim22k]

Separate structuralize pass logic to handle two cases:

1. Rewrite as generic version
2. Handle very simple custom versions

Other custom versions will be left unchanged and will need to be handled by another lowering pass which only looks for those specific custom versions.

For example, ReduceToVector in the structuralize pass doesn't handle custom aggregators (count, argmin, first, etc). All it does is convert non-custom aggregators into ReduceToVectorGeneric. The normal lowering pass will then handle the generic version and separately the custom version.

All DWIM checks are now handled exclusively by the structuralize pass.

[coveralls]

Pull Request Test Coverage Report for Build 1948008025

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.868%

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Totals
Change from base Build 1947565749:	0.0%
Covered Lines:	4563
Relevant Lines:	5314

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💛 - Coveralls