Fix xmlschema time parsing

[wintonpc]

• resolves #898
• fixed bug caused by assuming exactly 9 fractional digits
• passing seconds to Time.new as a Rational eliminates the need to fudge nanoseconds by 0.5
• the result is now == to that of Time.parse, even for insane precisions
• the new code is faster than the old code for typical numbers of fractional digits (by my measurements)
• removed unused ID definitions left over from https://github.com/ohler55/oj/pull/810