Relegated ckreads -> ckparity

Ckparity is pretty flawed in littlefs, for several reasons. The biggest
one being that we can't even reliably detect single-bit errors.

But! It can still provide an extra layer of safety in a system where you
don't care about the extra code/stack cost.

And, for ckreads, performance cost...

Performance isn't a big problem for parity-checking. We can assume
metadata tags are going to relatively small (and can be controlled by
fragment_size). But for data checksums, ckreads risks O(b^2) when
performing many small reads, which can be a bit of a problem.

And since ckreads doesn't really prove anything interesting about the
system anymore, it makes sense to unbundle these two checks, rename
ckreads -> ckparity, and limit it to only checking parity bits.

This way, you can enable ckparity for a bit of extra safety, with a
code/stack cost hit, but without sacrificing performance.

---

I was hoping more code/stack savings, but since we still need to track
parity context in lfsr_data_t, and still need to intercept bd_read/cmp/
cpy calls that reference metadata, we end up needing to keep most of
the ck circuitry around:

                    code          stack
  default before:  36464           2672
  default after:   36464 (+0.0%)   2672 (+0.0%)

                    code          stack
  ckparity before: 38036           3080
  ckparity after:  38024 (-0.0%)   3080 (+0.0%)

We even end up still tracking checksum context for bptrs! Maybe we
should just go ahead and add ckcksums as a joke...