Left recursion

So far I have not seen a need for left recursion, particularly because **pe** emits values in a flat list. But some people seem to really care about it, so this issue is for tracking its implementation.

Using the memo to escape the left-recursive loop looks promising, and this method was chosen for Python's new pegen parser. Guido wrote about it here: https://medium.com/@gvanrossum_83706/left-recursive-peg-grammars-65dab3c580e1