So that e.g. `bstr == <any string type>` works. We want `bstr == ustr`
to work because we intend those types to be interoperable. We also want
e.g. `bstr == "a_string"` to work because we want bstr to be
interoperable with standard strings. In general we want to have full
automatic interoperability with all string types, so that e.g. `bstr == X`
works for X being all bstr, ustr, unicode, bytes (and later bytearray).

For now we add support only for comparison operators. But later, we
will be adding support for e.g. +, string methods, etc - and in all
those operations we will be following the same approach: to have
automatic interoperability with all string types out of the box.

The text added to README reflects this.

The patch to unicode.tp_richcompare on py2 illustrates our approach to
adjust builtin types when absolutely needed. In this particular case
original builtin unicode.__eq__(unicode, bstr) is always returning False
for non-ASCII bstr even despite bstr having .__unicode__() method. Our
adjustment is non-intrusive - we adjust unicode behaviour only wrt bstr
and it stays exactly the same as before wrt all other types.

We anyway do that with care and add a test that verifies that behaviour
of what we patched stays unaffected when used outside of bstr/ustr
context.