Switch to shasums

__NOTE: This is still work in progress!__

In lieu of https://github.com/sstephenson/ruby-build/issues/548, I decided to see how much work was involved in actually changing from MD5 checksums to SHA2 checksums; the current state of this PR is that the work is about halfway done.

The end-user experience should be working as expected, but I've not yet looked at the S3 mirroring stuff, and there are still broken tests. Most noticeably, the "[..]no shasum support" test in checksum.bats is broken, and I've been unable to figure out how to fix it. Any and all help is highly appreciated.

Feel free to tear this apart, but I'd prefer constructive criticism though :heart: