The LastBoundary methods seems inconsistent as to
where it reports a boundary. When called with a string
containing a single (non-combining) character, the boundary
is sometimes reported at the start of the string
and sometimes at the end:

This code:

    fmt.Printf("%d\n", norm.NFD.LastBoundary([]byte("a")))
    fmt.Printf("%d\n", norm.NFD.LastBoundary([]byte("♔")))

print 0 and 3. I would expect it to print 0 and 0 or 1 and 3.

Perhaps it would be better if neither FirstBoundary (LastBoundary)
never reported a boundary at the start (end) of a string.
That way it becomes easier to use FirstBoundary and LastBoundary
to iterate through strings when use of Iter is not convenient.