Allow getting an arbitrary length with a maximum upper bound

Sometimes you don't want more than N things generated because, for example, it makes the fuzz target too slow and run up against libfuzzer timeouts.

A solution to this is pretty similar to a solution to #35 in that they both want access to (or something closer to) the underlying int_in_range from the end of the byte string.

impl Unstructured<'_> {
    pub fn arbitrary_len_with_max<ElementType>(&mut self, max: usize) -> Result<usize>
    where
        ElementType: Arbitrary,
    {
        ...
    }
}

Start by reading the Unstructured API and the underlying int_in_range logic referenced in the issue, then compare the proposed approach with issue #35. Done means exposing an arbitrary-length operation that accepts a maximum and returns a length no greater than that bound, with behavior covered by the relevant existing tests.