XPath: Optimize attribute axis lookup

When looking for an attribute by name, finding the first attribute means
we can stop looking since attribute names are unique. This makes some
queries faster by 40%.

Another very common pattern in XPath queries is finding an attribute with
a specified value using a predicate (@name = 'value'). While we perform an
optimal amount of traversal in that case, there is a substantial overhead
with evaluating the nodes, saving and restoring the stack state, pushing
the attribute node into a set, etc. Detecting this pattern allows us to
use optimized code, resulting in up to 2x speedup for some queries.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1061 99668b35-9821-0410-8761-19e4c4f06640