XPath eval and dump to string

[dmalkr]

Additional bindings for eval XPath and output result node set to ByteString.

c14n package output result in XML canonical form with buggy output (strips node attributes and texts),
like this:

<div><x1/><x2/></div>

instead of

<div id="a1">txt1<x1/>txt2<x2/>txt3</div>

In this PR I introduce evalXPath function which evaluate XPath and then dump node set to ByteString.

So we can write: evalXPath "<div id="a1">txt1<x1/>txt2<x2/>txt3</div>" "//div".

This is fast implementation just to make proper benchmarks in InferXPath.

[mgajda]

Can you please add the unit test?