Make NativeString[Line] a distinct type

[jamadden]

Especially now that we document the implementation class of fields, having NativeString[Line] be simple aliases of either Bytes or Text[Line] is confusing and misleading, because what you see in the documentation is Bytes or Text, but that's not what you need to put in source code.

The idea would be to do something like

class NativeString(Text if PY3 else Bytes):
    """The native `str` type"""

We'd probably also want a INativeString interface...

I noticed the confusing documentation in zope.configuration.