Represent sizes as int64 instead of uint64

[ammario]

I think this package should accept and return sizes typed int64 instead of uint64.

The standard library uses int64 to represent sizes, so I do a lot of casting when using this package.

Thanks.

[dustin]

I understand the inconvenience, but that's a fairly large backwards incompatible change. Conceptually, the number of bytes in something is non-negative, so unsigned made sense at the time. Having to convert from int64 or int or float64 or whatever in order to format a number is an unfortunate language constraint, but it's not a logical flaw in the concept of representing bytes as non-negative numbers.

[ammario]

The breakage would be trivial to fix, and the orthogonality with the rest of the Go ecosystem would make this API significantly better. I can carry out the change, as well, if you reconsider this.

[nhooyr]

And with Go modules it doesn't have to be backwards incompatible.

[yvele]

Can this issue be reconsidered?