slicing string literals is broken

[Stefan Behnel]

I added a test case "run/literalslicing.pyx" to show that slicing string literals is broken. Basically, when you do

py_result = "abc"[2]

Cython will generate C code that accesses the C byte string directly and retrieves the second char in it. Fast and simple, but this triggers two bugs.

1) The result is a C char, not a Python string. Converting it to a Python object will return an int instead of a single-character string.

2) Doing the same with a unicode string will access the (multi-byte) UTF-8 representation of the string and thus return the wrong byte if the string contains non-ASCII characters.

[Stefan Behnel]

Regarding the unicode issue, I think it might be better to have a dedicated unicode type after all. That would allow us to give it non-C semantics while keeping the fast C semantics up for byte strings.