Change IO#write, IO#skip, IO.copy to return Int64

[bcardiff]

This PR

• Updates the unreleased changes in Make IO#skip IO#write returns the number of bytes it skipped/written #9233 to return Int64 instead of UInt64.
• Changes IO.copy to return Int64 for consistency
• Updates IO::Stapled#write_byte, String::Builder#write_byte which were missing in Make IO#skip IO#write returns the number of bytes it skipped/written #9233.

The dilema between using UInt64 or Int64 was due to:

• OS / libc uses unsigned int under the hood in some places.
  • Odly, I was sure LibC.write returned unsigned, but I was wrong 🤷‍♂️. I know we are not using the turned value directly in IO#write, but they are related.
• Keeping consistency with existing IO.copy

But since mixing signed and unsigned types will impose some performance issues, we can avoid them if we discourage unsigned int in some cases like this one.

[Sija]

Heads up: it seems that this change broke sth on nightlies - see https://travis-ci.com/github/Sija/blurhash.cr/jobs/341785188

[bcardiff]

@Sija not by this PR. https://travis-ci.com/github/Sija/blurhash.cr/jobs/340619464 is also broken in nightlies. Some of the dependencies is still not up to date with IO breaking changes.