crucible: add Task class

We need a mechanism for distributing work across processor cores and
disks.

Task implements a simple FIFO/LIFO queue model for executing closures.
Some locking primitives are included (mutex and barrier).

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

lib/Makefile

@@ -14,6 +14,7 @@ OBJS = \
 	path.o \
 	process.o \
 	string.o \
+	task.o \
 	time.o \
 	uuid.o \
 	.version.o \