Adds:
- ast/CaseDispatchDefaultSelector: AST node for a "default:" selector
- ast/CaseDispatchGroup: AST node for a collection of selectors and the code to execute
- ast/CaseDispatchGroups: AST node holding all groups in a case statement
- ast/CaseDispatchSelectors: AST node holding all selectors for one group
- ast/CaseStatement: AST node for a whole case statement
- codegen/CaseDispatchGroupVisitor: knows what to put in a case jump dispatch table
- codegen/CaseJumpDataBlock: knows how to build the entries in the data block
Also, detect multiple case selectors with the same value (or default). This is different from the OG compiler, which only detected duplicate `default:` selectors.
