vinyl: incorporate tuple comparison hints into vinyl data structures

Apart from speeding up statement comparisons and hence index lookups,
this is also a prerequisite for multikey indexes, which will reuse tuple
comparison hints as offsets in indexed arrays.

Albeit huge, this patch is pretty straightforward - all it does is
replace struct tuple with struct vy_entry (which is tuple + hint pair)
practically everywhere in the code. Now statements are stored and
compared without hints only in a few places, primarily at the very top
level. Hints are also computed at the top level so it should be pretty
easy to replace them with multikey offsets when the time comes.