feat(size): handle content size info

[zkat]

This is kind of a rough sketch of keeping size information in the cache index. Even though we already verify size if it's passed in, this can make error reporting a little nicer, as well as improve on the sorts of things verify() can do (and report!).

Having size information is super handy for being able to determine whether to, say, load the entire contents of something into memory vs using streams, figuring out whether to memoize them, etc. Both make-fetch-happen and cacache itself are able to take advantage of this.

This bumps the index version to 5, too, but as usual, that's not a breaking change.

I mentioned before that this could help with SHAttered, but I didn't realize both of them were the exact same size :< -- there's a separate mitigation for that to prevent it from affecting the cache if both are inserted.