Fix use def issues

[water111]

This fixes two bugs that appear to fix all known use-def issues.

First, it fixes a bug where attempted structuring on the same vertex multiple times, resulting in env updates happening twice. Now these are cached. This was harmless but triggered error.

The second bug is a little more complicated. The structuring pass marks the (set! x #f) in the delay slot as not a def, and leaves the defs as the actual expressions that write. Of course, we may not find one in all cases because of move elimination.

This standardizes on the last non-eliminated set, not in a delay slot, as the result variable to use for a cond without an else.

We could possibly use something else if detecting this causes trouble, but this was easy to implement and easy to understand the debug output.

[coveralls]

Pull Request Test Coverage Report for Build 974841140

• 39 of 39 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 69.335%

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Totals
Change from base Build 974550887:	0.02%
Covered Lines:	36774
Relevant Lines:	53038

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💛 - Coveralls