Moved stuff from string.rst to the examples directory.

docs/examples/tutorial/string/const.pyx

+cdef extern from "someheader.h":
+    ctypedef const char specialChar
+    int process_string(const char* s)
+    const unsigned char* look_up_cached_string(const unsigned char* key)

docs/examples/tutorial/string/const_char.pyx

+from libc.string cimport const_char, const_uchar
+
+cdef extern from "someheader.h":
+    ctypedef const_char specialChar
+    int process_string(const_char* s)
+    const_uchar* look_up_cached_string(const_uchar* key)

docs/examples/tutorial/string/const_left_out.pyx

+cdef extern from "someheader.h":
+    int process_string(char* s)   # note: looses API information!

docs/examples/tutorial/string/someheader.h

+typedef const char specialChar;
+int process_string(const char* s);
+const unsigned char* look_up_cached_string(const unsigned char* key);

docs/src/tutorial/strings.rst

@@ -300,20 +300,13 @@ Many C libraries use the ``const`` modifier in their API to declare
 that they will not modify a string, or to require that users must
 not modify a string they return, for example:
 
-.. code-block:: c
-
-    typedef const char specialChar;
-    int process_string(const char* s);
-    const unsigned char* look_up_cached_string(const unsigned char* key);
+.. literalinclude:: ../../examples/tutorial/string/someheader.h
 
 Since version 0.18, Cython has support for the ``const`` modifier in
 the language, so you can declare the above functions straight away as
-follows::
+follows:
 
-    cdef extern from "someheader.h":
-        ctypedef const char specialChar
-        int process_string(const char* s)
-        const unsigned char* look_up_cached_string(const unsigned char* key)
+.. literalinclude:: ../../examples/tutorial/string/const.pyx
 
 Previous versions required users to make the necessary declarations
 at a textual level.  If you need to support older Cython versions,
@@ -322,23 +315,17 @@ you can use the following approach.
 In general, for arguments of external C functions, the ``const``
 modifier does not matter and can be left out in the Cython
 declaration (e.g. in a .pxd file).  The C compiler will still do
-the right thing, even if you declare this to Cython::
+the right thing, even if you declare this to Cython:
 
-    cdef extern from "someheader.h":
-        int process_string(char* s)   # note: looses API information!
+.. literalinclude:: ../../examples/tutorial/string/const_left_out.pyx
 
 However, in most other situations, such as for return values and
 variables that use specifically typedef-ed API types, it does matter
 and the C compiler will emit at least a warning if used incorrectly.
 To help with this, you can use the type definitions in the
-``libc.string`` module, e.g.::
-
-    from libc.string cimport const_char, const_uchar
+``libc.string`` module, e.g.:
 
-    cdef extern from "someheader.h":
-        ctypedef const_char specialChar
-        int process_string(const_char* s)
-        const_uchar* look_up_cached_string(const_uchar* key)
+.. literalinclude:: ../../examples/tutorial/string/const_char.pyx
 
 Note: even if the API only uses ``const`` for function arguments,
 it is still preferable to properly declare them using these