Memory management

• sgs_DefaultMemFunc - example memory management callback
• sgs_Memory - allocate/reallocate/free memory
• sgs_Malloc - allocate specified amount of memory
• sgs_Free - free a previously allocated memory pointer
• sgs_Realloc - allocate/reallocate/free memory (requires implicit sgs_Context* C)
• sgs_Alloc - allocate object (requires implicit sgs_Context* C)
• sgs_Alloc_n - allocate an array of objects (requires implicit sgs_Context* C)
• sgs_Alloc_a - allocate object + additional memory (requires implicit sgs_Context* C)
• sgs_Dealloc - free a previously allocated memory pointer (requires implicit sgs_Context* C)