vinyl: remove optimized comparators

A vinyl statement (vy_stmt struct) may represent either a tuple or a
key. We differentiate between the two kinds by statement type - we use
SELECT for keys and other types for tuples. This was done that way so
that we could pass both tuples and keys to a read iterator as a search
key. To avoid branching in comparators when the types of compared
statements are known in advance, we provide several comparators, each of
which expects certain statement types, e.g. a tuple and a key. Actually,
such a micro optimization looks like an overkill, because a typical
comparator is called by function pointer and has a lot of comparisons
in the code, see tuple_compare_slowpath for instance. Eliminating one
branch will hardly make the code perform better. At the same time, it
makes the code more difficult to write. Besides, once we remove nils
from statements read from disk (aka surrogate tuples), which will
ease implementation of multikey indexes, the number of places where
types of compared statements are known will diminish drastically.
That said, let's remove optimized comparators and always use
vy_stmt_compare, which checks types of compared statements and calls
the appropriate comparator.