Add caching of data-bind string functions

[brianmhunt]

No description provided.

[brianmhunt]

This may be a bit of a substantial rewrite because the Identifier.prototype.lookup_value has the $context/etc from the parser, and new parsers are created recursively with each $context.

In particular, we need to look at calls to get_value since they need to have $context and node passed in as arguments.

E.g. object_add_value (a function that adds defined properties to returned objects), it is evaluated whenever the property is accessed. It does not have easy access to the $context, which might change depending on where the object is being accessed.