objectstore/packed: Check pack/idx checksums here.

We previously had helpers in format/pack/checksum that checks
.pack/.idx-related checksums with []byte-based APIs. But it only really
makes sense to use those []byte-based APIs on mmap's (otherwise it'd be
horribly inefficient).

Since the packed object-store only needs to check that the .pack and
.idx trailer match, we move the relevant part into objectstore/packed.
The rest are deleted for now; we'll definitely need a streaming version
for the pack verification (when ingesting packfiles from the network)
(though we might just make it a streaming API (writer? reader? not
decided yet) that *produces* a hash, then verify it in the caller; this
way we could reuse the function in the pack-producing routines). The
others might get the []byte-based APIs back, or perhaps they too get
streaming APIs.

Remember that "reading objects from a packed object store",
"creating/writing packfiles", and "ingesting an incoming pack (which
usually involves creating an .idx for it)", are all very different
tasks.

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