Improve AST structure by increasing the opportunities for merging

[paulgazz]

If two subparsers are in the same production, and one reduces a symbol because that symbol has an empty production, the two subparsers will no longer be able to merge, even though they could have if not for the empty production reduction.

One way to solve this would be to take the grammar hierarchy into account when looking for merge opportunities, e.g., by checking whether the subparsers are in the production (and at the same level in the AST). Since checking for merges is expensive, the current optimization strategy does as many reduces as possible for each subparser before attempting a merge.