crucible: add cleanup class

Store a function (or closure) in an instance and invoke the function
from the destructor.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/Makefile

@@ -3,8 +3,9 @@ TAG := $(shell git describe --always --dirty || echo UNKNOWN)
 default: libcrucible.so
 
 OBJS = \
-	crc64.o \
 	chatter.o \
+	cleanup.o \
+	crc64.o \
 	error.o \
 	extentwalker.o \
 	fd.o \