Fix array/rfc6962 case.

We were creating a node with a single branch.  Just weird.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Merge branch 'array'

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

Conflicts:
	test-trees.c
Alternate mmr_proof_len_fast, which matches construction.

Not as cool as maaku's version, but works.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
mmr: remove tree construction, mark it as fast.

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