Rename package to `archives`

[mholt]

The semantics/scope of this project has moved slightly since its inception, when it was originally intended to write archive files. Hence, archiver made sense as a name.

But now it reads, writes, inserts, and even abstracts away archives as file systems! Given that Go convention is for package names to represent a concept, rather than a verb/etc., it feels like I could name this better.

I will probably rename or release this package as archives with a v1.0, instead of releasing archiver v4.0. <4.0 already isn't supported anymore, so I don't really see any point in keeping this repo around (could be quite harmful even).

Anyway, I expect mholt/archives will be created soon, and a v1.0.0 tag will be created shortly thereafter. Not sure yet whether I will rename/replace this repo, or archive it (heh) and create a new one.

[mholt]

This is now done here: https://github.com/mholt/archives

(So as not to break existing projects/users)

I will be archiving this repo soon, but I'm not quite ready to tag a stable 1.0 in the new repo yet. I want to verify some of the exported API through field testing.