Add continuation to conversion routines

This is necessary to make sure that conversion functions use the correct scheme-- will be necessary when we have more than one scheme. Also gives them the chance to pass flags to Convert().
Also, I'm not sure I like Continue, because you're not really using it like a continuation (in the sense of binding a partially executed function into another function) you're really using it more like a converter, or a scope.  Perhaps ConversionScope?  CurrentConverter?

Anyway, I don't think Continue is right.