source-gen: Add RedHatTransformer import for abstract classes

A bit late now, but this has been bugging me for a while
Add ElseCaseClause (ElseClause in RedHat AST) for #61
Add tests for ElseCaseClause backend conversion
Add CaseClause for #61
Add tests for CaseClause backend conversion
Add SwitchCases (RedHat’s SwitchCaseList) for #61
Add tests for SwitchCases backend conversion
Add SwitchClause for #61
Add tests for SwitchClause backend conversion
Add SwitchCaseElse (RedHat’s SwitchStatement)

Implements and closes #61.
Add tests for SwitchCaseElse backend conversion