tal: append the length property to the initial allocation.

tal: append the length property to the initial allocation.

Since we never post-add a length property, we can save some cycles by
allocating it together with the object itself:

Before:
	$ ./samba-allocs talloc.dump --tal
	Tal time:                6058997-6215211(6102645)ns
	Tal_free time:           4791112-4968964(4824814)ns
	Single tal_free time:    3173647-3331584(3200183)ns
	$ ./samba-allocs talloc.dump --tal-size
	Virtual size = 9809920, RSS = 3952640

After:
	$ ./samba-allocs talloc.dump --tal
	Tal time:                5911994-6043808(5956914)ns
	Tal_free time:           4676026-4834598(4719908)ns
	Single tal_free time:    2888195-3067365(2922298)ns
	$ ./samba-allocs talloc.dump --tal-size
	Virtual size = 9809920, RSS = 3948544

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