Initial pass at removing the old builtin string

Removed everything I could find in the compiler that dealt with the
string type. I have been adding things back as needed, mostly related to
coercion and string literals.

modules/internal/String.chpl has been renamed to CString.chpl and now
only contains c_string related code.

modules/standard/NewString.chpl is now modules/internal/String.chpl and
defines the new standard string type.

Various other changes in the modules trying to get everything to play
nicely.
c_string instead of string