Prefix headers with 'ctypes_'

This change prefixes all headers with `ctypes_` to avoid collisions.  The principal advantage is in simplifying the set of flags needed when compiling.  Whereas you might previously have written something like the following to build generated C files

```
ocamlfind opt -I $(ocamlfind query ctypes)/.. foo.c
```

you can now instead write

```
ocamlfind opt -package ctypes.stubs foo.c
```

Unfortunately this breaks backwards compatibility, but the breakage can be fixed by a simple tweak to the build system and the result will usually be simpler.

Closes #225.