Introduce SwitchLike as a supertype of SwitchExpression and SwitchStatement (since they have the same grammar). Pull up all interesting methods

Reorganise structure of the block:

• A SwitchLike is an Iterable<ASTSwitchBranch>
• A SwitchBranch is either a SwitchFallthroughBranch or a SwitchArrowBranch (the latter replaced SwitchLabeledRule)

About SwitchArrowBranch:

• Previously we distinguished several kinds of SwitchLabeledRule
• That's annoying because they don't provide more specialized API: the only difference is on the right-hand-side of the arrow.
• So SwitchArrowBranch merges those three alternatives, and the three nodes that may appear as RHS share a common interface now

About SwitchFallthroughBranch: it doesn't make much sense to make the statements children of the branches (they don't belong more to one branch than to the previous branches, which may fallthrough to them).... but at least it allows sharing a regular API with SwitchArrowBranch. In any case we're going to need some basic CFG to work with exit points in switches... TBD