XPath: Optimize constant filters/predicates

If a filter/predicate expression is a constant, we don't need to evaluate it
for every nodeset element - we can evaluate it once and pick the right element
or keep/discard the entire collection.

If the expression is 1, we can early out on first node when evaluating the
node set - queries like following::item[1] are now significantly faster.

Additionally this change refactors filters/predicates to have additional
metadata describing the expression type in _test field that is filled during
optimization.

Note that predicate_constant selection right now is very simple (but captures
most common use cases except for maybe [last()]).
XPath: Unify ast_filter/ast_predicate structure

Now expression is always _right for filter/predicate nodes to make optimize()
simpler. Additionally we now use predicate metadata to make is_posinv_step()
faster.

This introduces a weak ordering dependency in rewrite rules to optimize() -
classification has to be performed before other optimizations.
XPath: Only recognize numeric constant expressions

Numeric and boolean constant expressions in filters are different in that
to evaluate numeric expressions we need a sorted order, but to evaluate
boolean expressions we don't. The previously implemented optimization adds
an extra sorting step for constant boolean filters that will be more expensive
than redundant computations.

Since constant booleans are sort of an edge case, don't do this optimization.
This allows us to simplify apply_predicate_const to only handle numbers.