expose itos buffer size as a constant

expose itos buffer size as a constant

Elsewhere in Graphviz we sometimes need to print ints into fixed sized buffers,
but not using itos. So it is useful to have a value for the number of bytes this
requires that is also regularly validated by the Graphviz test suite. That is,
uses of this constant should be robust to future environment/platform changes,
because the itos tests will fail, prompting an update of this constant, that
will then correctly affect all usages of this.