Add new PyStr wrapper type

[cjdoris]

Has fields py::Py, ptr::Ptr{UInt8}, ncodeunits::Int where the latter two we get from https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_AsUTF8AndSize.

Can implement AbstractString easily on top of this by reusing bits of the Utils.StaticString implementation.