Backend for compact node-family representation.

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.

src/core/include.am

@@ -106,6 +106,7 @@ LIBTOR_APP_A_SOURCES = 				\
 	src/feature/nodelist/microdesc.c	\
 	src/feature/nodelist/networkstatus.c	\
 	src/feature/nodelist/nickname.c		\
+	src/feature/nodelist/nodefamily.c	\
 	src/feature/nodelist/nodelist.c		\
 	src/feature/nodelist/node_select.c	\
 	src/feature/nodelist/routerinfo.c	\
@@ -340,6 +341,8 @@ noinst_HEADERS +=					\
 	src/feature/nodelist/networkstatus_voter_info_st.h	\
 	src/feature/nodelist/nickname.h			\
 	src/feature/nodelist/node_st.h			\
+	src/feature/nodelist/nodefamily.h	        \
+	src/feature/nodelist/nodefamily_st.h    	\
 	src/feature/nodelist/nodelist.h			\
 	src/feature/nodelist/node_select.h		\
 	src/feature/nodelist/routerinfo.h		\