support memory context for arrays

[hugoArregui]

[nothings]

hash table had a context pointer because the hash table already has a lot of size overhead. however, the array size overhead is a lot smaller, so adding a context pointer to it adds a significant amount.

[hugoArregui]

@nothings would this makes sense if I add a #ifdef STBDS_ARRAY_CUSTOM_CONTEXT and only include the pointer in the structure then? If you have another idea I can try it as well (I though this may be useful for you as well because of the TODO comment in the file)

[nothings]

Oh, I forgot about the todo. I guess it's ok then.