py: Never intern data of large string/bytes object; add relevant tests.

Previously to this patch all constant string/bytes objects were
interned by the compiler, and this lead to crashes when the qstr was too
long (noticeable now that qstr length storage defaults to 1 byte).

With this patch, long string/bytes objects are never interned, and are
referenced directly as constant objects within generated code using
load_const_obj.