Size array builders at creation time.
Eliminate more uses of `var` in place of explicit types.
Use Hash.Combine

- Move `PartialCaseDecision` inside `DecisionDagBuilder`.
- The implementation of equality for `BoundDagEvaluation` and `BoundDagTemp` are moved to separate source files.
- `SyntheticBoundNodeFactory.AssignmentExpression(LocalSymbol left, BoundExpression right)` is removed as unused.
- Removed commented-out code in `LocalRewriter_Patterns.cs` and `PatternParsingTests.cs`