wasm: Maintain rightmost failure position during parse

[pdubroy]

• Add a global (rightmostFailurePos) to track the rightmost failure position
• Rule eval functions maintain the value in a local (failurePos) which is initialized from the global, and pushed back (if applicable) at the end of the rule body.
• Changes the representation of the memo entries and CST nodes, to allow failurePos to be memoized.
• Add a property-based test (using fast-check) for getRightmostFailurePosition