State supers: ByteString, MutableString, MutableByteArray
protected id init (pointer, int) (p, num);
id initCopy (pointer, int) (p, num);
In addition to what our super does, adjust our (new) capacity to fit our length.
void set char c at int index;
Set the byte at index to the character c, converted to a byte according to the default_encoding.
void add char c;
Add the byte encoding of the char c to this string.
ByteString substring (int, int) (start, len);
Override the ByteString implementation of this substring method, since that actually employs ByteSubstring objects which we can't use.