Optimization: Merge anonymous terminals

It's possible to improve Lark's speed, for all algorithms, by performing grammar simplification. Specifically, it's possible to merge adjacent anonymous terminals together, to reduce the parser work load, without affecting the resulting tree (An example of this technique can be seen in https://github.com/lark-parser/lark/issues/218)

In the same vein, ignored terminals can be joined to anonymous ones, with a similar effect.

Start by reading the technique described in issue #218 and locating Lark's grammar-construction path. Determine how adjacent anonymous terminals and ignored terminals joined to anonymous terminals can be simplified, then verify that resulting trees are unchanged and parser work is reduced.

Written by the indexing model from the issue text.