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>

test/Makefile

@@ -5,6 +5,7 @@ PROGRAMS = \
 	limits \
 	path \
 	process \
+	task \
 
 all: test