otherwise, different ways of spelling special characters can end up being correctly escaped or not in the C file

fixed unicode escape handling in byte strings
unescape \xXY in string literals as C allows it to conflict with trailing hex numbers - output string escaping will do the right thing

fix format string in __Pyx_AddTraceback() to let it match input types (line number is not defined as unsigned)

quick way to enable multiple compilation *without* reusing contexts (still about 10x faster than separate startup)

add a warning for unraisable exceptions (but do not show it by default, as this is often irrelevant)