Fix the SafeEncoding behaviour

Currently we have bad behaviour in SafeEncode:
  - binary strings are actually not handled correctly (ahem)
  - the encoding is not stable, due to use of string_escape

For this reason, we replace the use of string_escape with part of the
code of string escape (PyString_Repr in Objects/stringobject.c); we
don't escape backslashes or single quotes, since that is that makes it
nonstable. Furthermore, we only use the encode('ascii', ...) for unicode
inputs.

The patch also adds unittests for the function that test basic
behaviour.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>