Storage-plus: See if we can simplify PkOwned to Vec<u8>

[ethanfrey]

Is there a reason why we don't use that directly like &[u8]. Maybe an oversight when I wrote this API?

Then we can just return Vec<u8> than forcing wrapping.

[maurolacy]

Using &[u8] or &str directly might be possible, but will require to carefully track and fix lifetime conficts.

A simpler way seems to be implementing needed traits (PrimaryKey, Prefixer, probably others) directly for Vec[u8], and remove PkOwned entirely.

[maurolacy]

Created #233 to track / fix the lifetimes issue.