Update changelog & fix version-headings
Update changelog & fix version-headings

The entries for the 1.2.3.0 release have been reordered according to
their importance.

The version numbers follow the same heading style as `aeson`'s
changelog (i.e. heading level matches severity of version bump)

bos/text,bgamari/text,bgamari/text,bgamari/text,bos/text,bos/text,bos/text,bgamari/text