Convert Python bytes and strings to Elixir binaries without copying

[jonatanklosko]

bytes are immutable and PyBytes_AsStringAndSize returns pointer to the buffer. Similarly str are immutable and PyUnicode_AsUTF8AndSize returns a pointer to cached buffer that lives as long as the object.

Currently we create a new Erlang binary, which means copying the data once. This PR changes it such that we create resource around the Python objects and create a zero-copy resource binary term.