Improve parameters parsing and fix callbacks

[albestro]

Close #117

As per discussion in the issue, I tried to improve a bit the parser for parameters. It is far from being perfect.

The new method should is not regex based, it should be able to detect all types and pointers with related constness. The constness information is stored inside Par as a list of bools, intrinsically giving two information:

• it is a value (len==0), a pointer (len==1) or a pointer to pointer (len==2)
• constness for each existing level of indirection

e.g.

• const int * -> [True, False]
• const int * const -> [True, True]
• int * const -> [False, True]

[albestro]

For the moment I worked to be able to reproduce (more or less) the same generated wrappers.

Here it is a diff between what I generated with master 943e80a and this branch

1579c1579
<     MediaReadCb = ctypes.CFUNCTYPE(ctypes.POINTER(ctypes.c_ssize_t), ctypes.c_void_p, ctypes.POINTER(ctypes.c_char), ctypes.c_size_t)
---
>     MediaReadCb = ctypes.CFUNCTYPE(ctypes.POINTER(ctypes.c_ssize_t), ctypes.c_void_p, ctypes.c_char_p, ctypes.c_size_t)
5876c5876
<                     ctypes.c_int, Media, MediaSlaveType, ctypes.c_uint, ctypes.c_char_p)
---
>                     ctypes.c_int, Media, MediaSlaveType, ctypes.c_int, ctypes.c_char_p)
5902c5902
<                     ctypes.c_uint, Media, ctypes.POINTER(ctypes.POINTER(MediaSlave)))
---
>                     ctypes.c_int, Media, ctypes.POINTER(ctypes.POINTER(MediaSlave)))
5913c5913
<                     None, ctypes.POINTER(MediaSlave), ctypes.c_uint)
---
>                     None, ctypes.POINTER(MediaSlave), ctypes.c_int)
7250c7250
<                     None, MediaPlayer, Position, ctypes.c_uint)
---
>                     None, MediaPlayer, Position, ctypes.c_int)
7659c7659
<                     ctypes.c_int, MediaPlayer, ctypes.c_uint, ctypes.c_char_p, ctypes.c_uint, ctypes.c_uint)
---
>                     ctypes.c_int, MediaPlayer, ctypes.c_uint, ctypes.c_char_p, ctypes.c_int, ctypes.c_int)

Some highlights:

• some int are now recognized as unsigned int (added the new type mapping)
• added in type2class a "wrong" mapping for unsigned char* to not alter too much the wrapper (at least for the moment)
• added in type2class_out the "right" mapping unsigned char* to ctypes.POINTER(ctypes.c_char) to make the callback media_read_cb get the right wrapper function signature
• in Par.flags member function, forced unsigned char* to be recognized as Flag.Out (for the same reason of the previous point)

[oaubert]

Great work @albestro , thanks. It all looks good to me, apart from the small remark in the review. Could you fix this, so that I can simply merge the pull request as-is? The cherry on the cake would be to add a test in the tests/test.py file if it you can figure out one, but this is more optional.

[albestro]

I've refactored the parse_param function by moving it to class Par and making it a @classmethod function.

Moreover, I implemented some very basic tests for parse_param and I improved it.

There is also a minor fix in another test.

[oaubert]

Awesome contribution, many thanks. I just merged it.