In DomainSpecificString, override __repr__ in addition to __str__

For some reason, string interpolation on a DomainSpecificString object
like "%r" % (domainSpecificStringObj) fails under PyPy, because the
default __repr__ implementation wants to iterate over the object. I'm
not sure why that happens, but overriding __repr__ instead of __str__
fixes this problem, and is arguably the more appropriate thing to do
anyways.