Added Merkle Mountain Ranges (MMR) as a tree type.
"Array" is a bit ambiguous as there are multiple ways of constructing a tree from an array, and in particular bitcoin uses a different method. What is actually being tested here is a tree of the structure specified by RFC 6962.
More single backlink variants.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Remove backlinks, since they weren't competitive.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Merge branch 'maaku-mmr'
Merge branch 'rfc6962' of git://github.com/maaku/rusty-junkcode

Conflicts:
	test-trees.c
Use naive build-and-measure implementation of MMR trees.

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