Prop298 says that family entries should be formatted with
$hexids in uppercase, nicknames in lower case, $hexid~names
truncated, and everything sorted lexically. These changes implement
that ordering for nodefamily.c.
We don't _strictly speaking_ need to nodefamily.c formatting use
this for prop298 microdesc generation, but it seems silly to have
two separate canonicalization algorithms.
This representation is meant to save memory in microdescriptors --
we can't use it in routerinfo_t yet, since those families need to be
encoded losslessly for directory voting to work.
This representation saves memory in three ways:
1. It uses only one allocation per family. (The old way used a
smartlist (2 allocs) plus one strdup per entry.)
2. It stores identity digests in binary, not hex.
3. It keeps families in a canonical format, memoizes, and
reference-counts them.
Part of #27359.
