This class gives access to the lowest level for arrays. As any low level array, you can
get high performances with NATIVE_ARRAYs, but you loose most valid bounds checks (as
you can do in plain C code for example).
Note: this class is the basic support for most of our high-level arrays-like classes: STRING,
ARRAY, FAST_ARRAY as well as MUTABLE_BIG_INTEGER. Each class using some attribute of some
NATIVE_ARRAY type needs an attribute named capacity with value set to the size of the
actual NATIVE_ARRAY. Value has to be adjusted after each calloc/realloc/create_from.