basic: add RB-Tree implementation

This adds an self-standing RB-Tree implementation to src/basic/. This
will be needed for NSEC RR lookups, since we need "close lookups", which
hashmaps (not even ordered-hashmaps) can give us in reasonable time.

.gitignore

@@ -246,6 +246,7 @@
 /test-pty
 /test-qcow2
 /test-ratelimit
+/test-rbtree
 /test-replace-var
 /test-resolve
 /test-ring

Makefile.am

@@ -766,6 +766,8 @@ libbasic_la_SOURCES = \
 	src/basic/missing.h \
 	src/basic/capability-util.c \
 	src/basic/capability-util.h \
+	src/basic/c-rbtree.c \
+	src/basic/c-rbtree.h \
 	src/basic/conf-files.c \
 	src/basic/conf-files.h \
 	src/basic/stdio-util.h \
@@ -1493,6 +1495,7 @@ tests += \
 	test-copy \
 	test-cap-list \
 	test-sigbus \
+	test-rbtree \
 	test-verbs \
 	test-af-list \
 	test-arphrd-list \
@@ -1728,6 +1731,12 @@ test_sigbus_SOURCES = \
 test_sigbus_LDADD = \
 	libshared.la
 
+test_rbtree_SOURCES = \
+	src/test/test-rbtree.c
+
+test_rbtree_LDADD = \
+	libshared.la
+
 test_condition_SOURCES = \
 	src/test/test-condition.c