make src into a range and get rid of count

[oconnor663]

This is an example implementation of a change @SimonSapin suggested in rust-lang/rust#53652 (comment).

[SimonSapin]

(Note that this is ~half of my suggestion, I also proposed changing dest from usize to RangeFrom<usize> to make it clear that it is the start position of some range. (However it is not dest.start()..buffer.len() which is the usual meaning of RangeFrom, so maybe that’s not such a good idea…)

[oconnor663]

Here's an example instantiation to look at the difference: https://godbolt.org/z/PsxH7K

It seems like a few instructions get added to the caller in this case, probably too small of a difference to be measurable?

[oconnor663]

I wonder if this is technically correct. If a slice actually ends at the largest possible address (I think that's one less than the maximum representable, to allow C to compute the p + len pointer?), then an inclusive range that passes the end of the slice by one would be converted from the panic it should be to a valid range one element shorter than the caller asked for.

[oconnor663]

Playing with this a little more, I think it's obviously better to read and write than my original API. I'm going to land it here and update the Rust PR.