Revert to RFC code: use punycode_uint and enum punycode_status.

Revert to RFC code: use punycode_uint and enum punycode_status.
However, we do continue to use size_t to promote use of sizeof() by
application writers.  We return punycode_bad_input if a size_t value
exceeds the largest punycode_uint, since the code presumably cannot
handle that.  If size_t is smaller than punycode_uint, it seems
unlikely that an application could be able to pass a string that is
longer than SIZE_MAX anyway, but should it happen the worst that occur
is that the punycode code will only see the beginning of the string,
which isn't unreasonable on a platform with a small SIZE_MAX.