Sequence comparison (starts, ends, contains) - issues 94, 96 - #222

Some afterthoughts:

• the examples using fn:compare as the callback function are wrong because fn:compare doesn't return a boolean
• the examples all use the markup conventions for executable examples but some are not executable because they have unstated context dependencies
• perhaps a better formalisation for contains-sequence($x, $y, $compare) would be starts-with-sequence($x, $y, $compare) or (exists(tail($x)) and contains-sequence(tail($x), $y, $compare)). Though it relies on short-cutting and/or for non-termination!
• there's also an out-by-one bug in the current formalisation of contains-sequence. If the first sequence has length 5 and the second has length 2, then a match can occur at positions 1, 2, 3, or 4. However, testing at all positions does no harm, since subsequence() at later positions will return a sequence that is too short to satisfy the test.

Resolved by PR #244, a fixed version of this PR