only write requested chroms to d4 file

[cademirch]

hi @brentp sorry i'm back again - I just noticed this when my per-base d4 files were way bigger than expected

I think this is the right fix? I haven't tested it since I haven't set up a env to build mosdepth locally

[brentp]

hts_nim_static_builder.gz
hi, if you're on linux and have docker, you can use this binary, (gunzip, chmod +x)
then run from mosdepth directory:

./hts_nim_static_builder -n mosdepth.nimble -s mosdepth.nim

and test the resulting mosdepth binary. It will get the needed docker image on the first build.