Introduce `CountedInput`. a wrapper which counts the number of bytes successfully read

[gui1117]

I saw some code which tries to track the number of byte read with remaining_len. But remaining_len don't always give information.

I think it is better to provide an accurate counter.

I used u64 because it made the code simpler. And more general purpose.

Otherwise I was thinking using a type:

enum Count {
    Exact(u32),
    MaxReached,
}

and do the count in u32.

But most machine being 64bit, I think it should execute good.

[shawntabrizi]

fyi we also have saturated_inc() which mutates in place and adds one.

[bkchr]

fyi we also have saturated_inc() which mutates in place and adds one.

No in the standard library (which is only used in this crate)