[Vala] Vala should use g_slice for structures.

[Tal Hadad <tal_hd hotmail com>]

Hello all

I've seen in my generated code, that Vala use g_slice for alloc. classes.
But on structs, Vala use g_new0 and g_free.

I think it should be also in structures "dup" and "free" funcs, for faster performance,
even known struct duplicating isn't used a lot.

Hope to hear your opinion.

Tal