Fix comparison of xs:unsignedLong to xs:decimal

[GuntherRademacher]

These queries currently return a wrong result:

xs:unsignedLong('3') eq 3.1,              (: true, but should be false :)
xs:unsignedLong(3) lt 3.1,                (: false, but should be true :)
compare(xs:unsignedLong('3'), 3.1)        (: 0, but should be -1       :)

This is caused by both Uln.equal and Uln.compare neglecting the fractional part when the right-hand-side operand is a Dec. This is fixed here.