Improve memory allocation for MifareDESFireKeys

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I'm currently writing Go bindings for the libfreefare.

Go provides garbage collection, Go programmers expect that objects which aren't 
tied to external resources do not need explicit deinitialization. A 
MifareDESFireKey is such an object. It contains a key used for initialization. 
As far as I am concerned, a MifareDESFireKey does not allocate any resources 
and can be free'd at any time.

Right now, MifareDESFireKeys are allocated by the libfreefare using malloc() 
and free(). This makes it very hard to give control over allocation to a 
garbage collector: You need two layers of wrapping which causes performance 
loss and an unneccessary amount of extra garbage to be allocated. Library 
support is needed to be able to control the allocation of MifareDESFireKeys.

I request to implement one of the following things:

1) Completely define struct mifare_desfire_key in freefare.h and for each 
function　mifare_desfire_*_key_new() provide an additional 
mifare_desfire_*_key_init() with the same effect, except that you explicitly 
pass a MifareDESFireKey.

2) Provide a function mifare_desfire_key_sizeof() that returns sizeof(struct 
mifare_desfire_key_sizeof) and otherwise proceed as in 1).

3) For each function mifare_desfire_*_key_new() provide an additional 
mifare_desfire_*_key_custom_new() with an extra parameter

    void *(*alloc)(size_t size)

to pass a custom memory allocation routine. Albeit not perfect, this would 
already simplify allocation control by a big deal. What is missing is the 
ability to embed a MifareDESFireKey into a custom Go struct to save even more 
space.

Original issue reported on code.google.com by fuz...@gmail.com on 25 Feb 2014 at 12:58