Records and missingness, redux

[kescobo]

At one point, there was a discussions about how to deal with sequence records and missingness.

The discussion, previously found at https://github.com/orgs/BioJulia/teams/developers/discussions/2, but that link no longer works. Some evidence of the content of the discussion remains in BioJulia/FASTX.jl#63, and @jakobnissen is going to reach out to github support to see if we can recover it. Untill then, perhaps we can try to recreate it here?