Rename Item::size() to byte_size().

This makes its intent clearer. It also allows usage of the method name size()
for osmium::memory::Collection<> classes where we want it for the number of
objects just as in STL collections.
Add size() methods to Tag/WayNode/RelationMemberList
Make test pass with picky (and correct) clang++
Add default value to get_value_by_key()
Add some missing includes