Use `TypeNode#union_types` where possible#5018

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Sija wants to merge 2 commits intocrystal-lang:masterfrom Sija:pr-4995-followup

+30-30

Commits on Mar 6, 2018

Replace hard-coded class name arrays with TypeNode#union_types calls to appropriate constants
Sija
committed
Remove (superfluous now) calls to AstNode#id
Sija
committed