Various updates and fixes

[h-2]

Important things to note:

• output handlers and writers are no longer required to be default-constructible
• output handlers may write headers in their destructors and may throw if no header was set
• compressions now have two sets of extensions (specific to format .gz but also "implied" ones like .bam or .bcf)
• the output iterator now has some utilities for writing types in their byte-representation (useful for binary formats)

Clang-format has moved some things around again, but the stable release of version 14 is in two weeks, so we should be fine with this soon (clang-format will be fixed at version 14 for now!).

@smehringer Can you have a look at all commits except "var_io/vcf/bcf" (because that is likely long and not so relevant for you)?

[h-2]

I hope it is OK that I merge this for now. I am very happy to accept a review of the existing code later on!