lib/core: remove ropes intrude import in stream

lib/core: remove ropes intrude import in stream

The intrude import of ropes in lib/core/stream was only due to the
ropification of the read_all_bytes.

As the leaves were too small to have a positive impact with the actual
threshold value being chosen here, we remove the ropification in the
read_all function.

This simplifies code a lot, and keeps out the intrude import of the
ropes module.

We also take advantage of this modification to start using codecs here
to decode the string.

Signed-off-by: Lucas Bajolet <lucas.bajolet@gmail.com>