The buff_size parameter is embedded in the string data according
to the STR_EXTRA_BUFFER bit. Another approach would have been to
reserve 4 additional bytes of overhead for buff_size. I decided
to take the approach of saving per-string overhead.
One disadvantage of embedding buff_size in the data is that, in
the case where buff_size - length < 5, there are not enough bytes
to store both the NULL and buff_size. In this case,
STR_EXTRA_BUFFER is cleared and 1 to 3 usable *data
bytes are lost in a memory hole. Understand that this is not the
same as a memory leak. A memory leak is a situation where a program
loses track of memory, leaving the memory inaccessible until the
program terminates. In this situation, a free(data) will
correctly return the bytes in the memory hole to the program. This
will happen when the string is destructed, when it is resized larger,
when it is compacted, or when it is manually reallocated by the user.