This updates the terminology everywhere - blocks become blobs and
`storage.Storage` becomes `blob.Storage`.
Also introduced blob.ID which is a specialized string type, that's
different from CABS block ID.
Also renamed CLI subcommands from `kopia storage` to `kopia blob`.
While at it introduced `block.ErrBlockNotFound` and
`object.ErrObjectNotFound` that do not leak from lower layers.
completely rewrote password storage:
- by default passwords are kept in OS-specific keyring (Keychain on macOS,
Windows Credentials Manager on Windows), which can be optionally disabled
to store password in a local file.
- on Linux keychain is disabled by default (does not work reliably
in terminal sessions), but can be enabled using command-line flag.
