* Cleanup of string initialization code for limited API

Now appears in a few large blocks rather than

```
 # if CYTHON_LIMITED_API
   individual string line
 # endif
```

* Fixed issue with some unicode strings, e.g.

```
# cython: language_level=3str, binding=True

def non_ascii_str():
    s = 'ø\x20\u0020'
    assert isinstance(s, str)
    return s
```