Renamed CSS axis tokens.

These have been renamed as following:

T_CHILD => T_GREATER
T_FOLLOWING => T_TILDE
T_FOLLOWING_DIRECT => T_PLUS
Support for parsing CSS axes.
Added benchmark for descendant-or-self
Fixed descendant-or-self with a predicate.

Processing of this axis along with a predicate wouldn't quite work out. Even if
the predicate returned false the node would still be matched (which should not
be the case).