Zero-Copy Slice Readers

[krakow10]

This splits off chunks of a byte slice instead of allocating a String or Vec where applicable. There are several different classes of changes here that are interlaced in the diff, but I have grouped similar changes by commit to make it easier to review the repetitive changes. Potentially 5-8% faster on the deserialize microbenchmarks, but I can't really trust my benchmarks.

[Dekkonot]

I'm a clever boy and can work out that the new functions aren't a part of RbxReadExt because they operate explicitly on a slice instead of a nebulous impl Read, but it might be worth writing it down for future generations. I'll leave that to your discretion, but I tend to err on the side of writing things down even if they feel obvious because it lowers the bus factor for the project.

Otherwise this seems fine to me.

[krakow10]

the new functions aren't a part of RbxReadExt ... it might be worth writing it down

Done. When I was writing this PR at first I was considering a trait too, but decided this was simpler. If you think you can improve the clarity of the documentation wording, suggestions or changes are welcome.