Cleanup and fix string initialization code for LIMITED_API (GH-3378)

* 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
```

Cython/Compiler/Code.py

@@ -1512,6 +1512,17 @@ class GlobalState(object):
             w = self.parts['pystring_table']
             w.putln("")
             w.putln("static __Pyx_StringTabEntry %s[] = {" % Naming.stringtab_cname)
+            w.putln("#if CYTHON_COMPILING_IN_LIMITED_API")
+            w_limited_writer = w.insertion_point()
+            w.putln("#else")
+            w_not_limited_writer = w.insertion_point()
+            w.putln("#endif")
+            decls_writer.putln("#if !CYTHON_COMPILING_IN_LIMITED_API")
+            not_limited_api_decls_writer = decls_writer.insertion_point()
+            decls_writer.putln("#endif")
+            init_globals.putln("#if CYTHON_COMPILING_IN_LIMITED_API")
+            init_globals_limited_api = init_globals.insertion_point()
+            init_globals.putln("#endif")
             for idx, py_string_args in enumerate(py_strings):
                 c_cname, _, py_string = py_string_args
                 if not py_string.is_str or not py_string.encoding or \
@@ -1530,23 +1541,20 @@ class GlobalState(object):
                     py_string.cname)
                 self.parts['module_state_traverse'].putln("Py_VISIT(traverse_module_state->%s);" %
                     py_string.cname)
-                decls_writer.putln("#if !CYTHON_COMPILING_IN_LIMITED_API")
-                decls_writer.putln(
+                not_limited_api_decls_writer.putln(
                     "static PyObject *%s;" % py_string.cname)
-                decls_writer.putln("#endif")
                 if py_string.py3str_cstring:
-                    w.putln("#if PY_MAJOR_VERSION >= 3")
-                    w.putln("{&%s, %s, sizeof(%s), %s, %d, %d, %d}," % (
+                    w_not_limited_writer.putln("#if PY_MAJOR_VERSION >= 3")
+                    w_not_limited_writer.putln("{&%s, %s, sizeof(%s), %s, %d, %d, %d}," % (
                         py_string.cname,
                         py_string.py3str_cstring.cname,
                         py_string.py3str_cstring.cname,
                         '0', 1, 0,
                         py_string.intern
                         ))
-                    w.putln("#else")
-
-                w.putln("#if CYTHON_COMPILING_IN_LIMITED_API")
-                w.putln("{0, %s, sizeof(%s), %s, %d, %d, %d}," % (
+                    w_not_limited_writer.putln("#else")
+                w_not_limited_writer.putln("{&%s, %s, sizeof(%s), %s, %d, %d, %d}," % (
+                    py_string.cname,
                     c_cname,
                     c_cname,
                     encoding,
@@ -1554,26 +1562,21 @@ class GlobalState(object):
                     py_string.is_str,
                     py_string.intern
                     ))
-                w.putln("#else")
-                w.putln("{&%s, %s, sizeof(%s), %s, %d, %d, %d}," % (
-                    py_string.cname,
-                    c_cname,
-                    c_cname,
-                    encoding,
+                if py_string.py3str_cstring:
+                    w_not_limited_writer.putln("#endif")
+                w_limited_writer.putln("{0, %s, sizeof(%s), %s, %d, %d, %d}," % (
+                    c_cname if not py_string.py3str_cstring else py_string.py3str_cstring.cname,
+                    c_cname if not py_string.py3str_cstring else py_string.py3str_cstring.cname,
+                    encoding if not py_string.py3str_cstring else '0',
                     py_string.is_unicode,
                     py_string.is_str,
                     py_string.intern
                     ))
-                w.putln("#endif")
-                init_globals.putln("#if CYTHON_COMPILING_IN_LIMITED_API")
-                init_globals.putln("if (__Pyx_InitString(%s[%d], &%s) < 0) %s;" % (
+                init_globals_limited_api.putln("if (__Pyx_InitString(%s[%d], &%s) < 0) %s;" % (
                     Naming.stringtab_cname,
                     idx,
                     py_string.cname,
                     init_globals.error_goto(self.module_pos)))
-                init_globals.putln("#endif")
-                if py_string.py3str_cstring:
-                    w.putln("#endif")
             w.putln("{0, 0, 0, 0, 0, 0, 0}")
             w.putln("};")