Added a String type to the Value package. Mostly this is to make it
possible to generate an answer checker for it that uses the parser.
It also makes it more consistent for handling things like stringify,
especially when they are recursively called internally.
Added new default strings "NONE" and "DNE", since these get used a
Modified how the default parameters are set up for the answer
checkers, and how the answer checker reports the expected answer type
in type mismatch error messages.