Pull out additional Generate logic into functions; add tests

[egibs]

Follow-up for #1055 with the promised tests. I pulled most of the remaining conditional logic into functions that are much easier to test and added tests for the respective functions and relocated the somewhat-redundant size/checksum calculation which can be done as part of the initial file scan.

For the checksum calculations, we can use MinIO's sha256-simd package which is quite a bit faster to eliminate some of the overhead.

I also continued the trend of using iota values instead of the direct integers since it may not be apparent that an integer is actually a severity.

Key points:

• The tests pass and should capture most of the states we care about
• The new functions caused no test data drift