Scope memo expansions to sub-branches rather than globally

[maetl]

This is an experimental new feature to enable memo expansions to be scoped to branches, meaning that the memos can be set multiple times for a single grammar run and will remain consistent throughout subtree expansions, but reset at higher levels of the tree.

Standard memo expansions are global, so the following grammar will always expand to the same repeating name:

{
  start: "{report};{report}",
  report: "{@name}:{description}",
  description: "{@name}",
  name: ["fluorite", "pyrite", "sodalite"]
}

result.text => "pyrite:pyrite;pyrite:pyrite"

What we want here is to reuse sub-branches with the same text patterns while changing the memoized value at the top level branch, so that we can repeat the same expansion multiple times and get different results across each top level branch, while keeping the memoization consistent within the branch.

{
  start: "{report};{report}",
  report: "{@@name}:{description}",
  description: "{@@name}",
  name: ["fluorite", "pyrite", "sodalite"]
}

result.text => "sodalite:sodalite;fluorite:fluorite"

There are some problems with this implementation so it can’t be merged as-is, but it’s hopefully a useful starting point for further discussion both around the context of the feature and syntax change, and about the implementation.

What I’d also like to consider is if some of this functionality can be moved from the registry into the syntax nodes so that some of the random selection details can be better encapsulated, rather than letting the registry drive nearly all the behaviour.