Add the PureXML.Schema namespace validating XSD Part 2 built-in datatypes
and constraining facets. Built-ins span string/boolean/decimal, the bounded
integer family with intrinsic bounds, float/double, duration, the eight
date/time types, hexBinary, base64Binary, anyURI, QName, and the
Name/NCName/NMTOKEN/language family. Facets: length/minLength/maxLength,
pattern (via the regex engine), enumeration (value-space comparison),
whiteSpace, min/maxInclusive/Exclusive, totalDigits, fractionDigits.
Decimals compare exactly with no floating-point loss; date/time fields are
range-validated with leap years, 24:00:00, and timezone bounds.
Closes#8
