#7 a wheat string type

[Mark Lentczner]

In place of pt::string Wheat should have its own string.
Properties that this class should have that string doesn't:

[] enforced Unicode and UTF-8 semantics
[] sharing of storage wherever possible
[] no allocation for construction from c-strings
[] not thread safe unless requested
[] allow internal nulls (do we need this? I'm not sure)
[] C++ class like syntax: aString->length() rather than pt::
length(aString)

Wheat would also need a string list (with a sorted option)
and templated string->X map class