Add crucible-verify-raw and crucible-raw-extent packages

[mkeeter]

This PR extracts the crucible-verify-raw tool from #1791. The on-disk data structures for raw extents are moved to a new crucible-raw-extent package, which is used by both crucible-downstairs and crucible-verify-raw.

[leftwo]

I was thinking about the "one binary to rule them all" vs. having separate tools.

Having one binary means it's already available on the sled, which I liked, and if we did not make it part of crucible-downstairs, we would have to add framework to build and install the new binary into the image so we would have it installed without having to copy it in from outside the rack.

However, it's actually not that hard to solve this another way. We already have the crucible-dtrace package that is part of every global zone, so we can stuff these tool binaries in this same package without too much additional work.

[mkeeter]

@leftwo maybe we should rename it crucible-utils instead of crucible-dtrace?

[leftwo]

@leftwo maybe we should rename it crucible-utils instead of crucible-dtrace?

Yeah, it's now becoming more that just DTrace. I have other things I want to stuff in there as well that are more "tools" than specific DTrace scripts.