Detect and remove unreachable parse tree nodes

In some cases, a nonterminal is used only for syntax, but its contents are irrelevant. In these cases, the parse tree node is unreachable from the root node. Instead of uselessly creating and immediately dropping a parse tree node, we should return unit.

EmitPtree: For this, we need to construct an AST representation on a higher level than the Caml AST we currently produce. A subsequent pass over this AST can remove unreachable types.

PtreeMaker: The module creating parse tree actions may need to know about unreachable types, as well.