multi-index verification

[Byron]

Currently multi-indices can't be verified, but there is nothing preventing this.

Tasks

• verify the file itself with its checksum. Reuse and deduplicate code from index/verify
• verify integrity by doing the same for each index the multi-file refers to
• add multi-index support to pack-verify
• add journey test for that and figure out a way to write it as JSON and for humans (probably after we can write multi-indices ourselves from a set of packs)