Remove homebrew-fork `Checksum` class and refactor

* perform actual checksum calculation within `download.rb`
* use an accessor named `sha256` instead of `sums` for checksum data
* migrate checksum-specific exceptions out of homebrew-fork

The `sums` accessor was a relic from when Casks could contain any of `md5`, `sha1`, etc.  The `sums` array was more extensible (if we change checksums again) but balancing against the clarity of naming accessors after the relevant stanzas, I opted for the latter.

The refactor is incomplete.  We may want to spin checksums out of `download.rb` and/or combine them with GPG checks.

cc @ndr-qef