tal: add benchmark based on Samba4's talloc usage.

tal: add benchmark based on Samba4's talloc usage.

This is based on a very simple dump of S4's talloc tree, which we try
to duplicate using tal.  The benchmarks are simply to allocate all the
nodes, free all the nodes individually, and a top-level free.

Size results (32-bit x86):
	$ ./samba-allocs talloc.dump --talloc-size
	Read 25998 nodes
	Virtual size = 10469376, RSS = 4759552
	$ ./samba-allocs talloc.dump --tal-size
	Read 25998 nodes
	Virtual size = 9629696, RSS = 3948544

Speed results:
	$ ./samba-allocs talloc.dump
	Read 25998 nodes
	Malloc time:             1912082ns
	Free time:               1384892ns
	Talloc time:             2851531ns
	talloc_free time:        2133801ns
	Single talloc_free time: 1696298ns
	Tal time:                2686952ns
	Tal_free time:           3316153ns
	Single tal_free time:    1639407ns

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>