Arbitrary memo access

This sounds like it might be a bad idea, but it could be very useful for an expression or an action to have direct access to the memo for storing or retrieving arbitrary information. The purpose is for expressions that accumulate information across different branches. It's something like persistent bound values.

I can imagine some expression like `foo@Bar` (syntax undecided) that stores the values of `Bar` in the memo at key `foo`, but that might not be enough. Perhaps the `Action` class can take the memo as another argument, then some `Memo` action does the same as the expression. Users could create subclasses of `Action` or `Memo` to do more than just store the value.

This would need some restrictions, such as only allowing string keys so they don't overwrite actual memo data.