Figure out how to handle labels that aren't valid paths

[woodruffw]

The fact that labels currently need to be valid paths (more precisely, valid base filenames) is a leaky abstraction. A user should be able to label their record with anything (within reason): a URL, slashes, etc.

[woodruffw]

Right now this will fail with a filesystem error:

$ kbs2 new "test/test"
? Username? foo
? Password? ********
Error: No such file or directory (os error 2)

[woodruffw]

It's not ideal for diffing, but maybe something like a single binary KV store makes the most sense at this point.

sled is probably a good option.