Replaces []Format with a Group type.
A bit more type safe.
Breaking change for RegisterFormat, now takes a first argument that is a "single" format group.
Lots of naming cleanup.
This is also preparation for decode group argument which will enable doing intresting
probing, ex a format decoder could know it's decode as part of probe group (html could
be probed possibly), or have "arg probe" group for decoder who inspect args to know
if they should probe (-d /path/to/schema etc) to enable nice CLI-ergonomics.
