Add NodeRef::location() function returning a reference.

Needed for updating the location.
Add NodeRef::set_ref() function. Make setters return *this.
Increase initial buffer size in PBF reader for improved performance.
Small improvement in XML parser. Attribute can't both be k and v.
Improve performance by removing strlen() calls.

strlen() was called for every user, tag key and value, and role in
a PBF file, but the length is available there already. Also adds
some documentation to the functions involved.