Evaluator should be public

[xian]

It should be possible to obtain and configure an Evaluator separately from Expressions, so different functions/constants/etc. can be configured.

(This will also require Expr and its various subclasses, probably Parser, and likely other interfaces and classes to be made public. I honestly don't see much reason to restrict visibility of much in the library, it's already nice and tight. IMO the easiest fix is just to make everything public.)