Small performance enhancements for cut: bytes cutting

Several small performance enhancements.

For very large, or infinite ranges, `uutils cut` bytes cutting is now 3 times faster than `GNU cut`.

    $ ranges=1-
    $ perf stat -r 10 build/cut -b $ranges `find /boot -type f -readable` >/tmp/uucut
    ...
    0.095898887 seconds time elapsed ( +-  0.14% )
    $ perf stat -r 10 cut -b $ranges `find /boot -type f -readable` >/tmp/gcut
    ...
    0.308146576 seconds time elapsed ( +-  0.24% )

    $ ranges=4-6,3-8,20-30,15-25,23,6
    $ perf stat -r 10 build/cut -b $ranges `find /boot -type f -readable` >/tmp/uucut
    ...
    0.102587877 seconds time elapsed ( +-  0.07% )
    $ perf stat -r 10 cut -b $ranges `find /boot -type f -readable` >/tmp/gcut
    ...
    0.179558780 seconds time elapsed ( +-  0.06% )

    $ ranges=-4097
    $ perf stat -r 10 build/cut -b $ranges `find /boot -type f -readable` >/tmp/uucut
    ...
    0.095640648 seconds time elapsed ( +-  0.13% )
    $ perf stat -r 10 cut -b $ranges `find /boot -type f -readable` >/tmp/gcut
    ...
    0.305291892 seconds time elapsed ( +-  0.24% )

For relative performance improvement see issue #345.
Sure, however

> 1. The code is in actuality safe (e.g. won't go out of bounds).

actually requires verification, and easily can be verified incorrectly: human error is killer.

For this code, the correct way to verify is ensuring that `self.start` and `self.end` are always in bounds. That's not obvious, you have to go and track down/follow all uses of the `self.start`/`self.end` through out this whole module, as they're not local to this function. (And is even worse below, where you need to be tracking `max_segment_len` too.)

Furthermore, there's [a pile of rules](http://doc.rust-lang.org/master/rust.html#behavior-considered-unsafe) that need to be upheld inside `unsafe`: breaking any of them is undefined behaviour and can lead to an arbitrarily broken program. `unsafe` should be your absolute last resort when optimising a program.