shachain: clarify design in terms of binary tree, reverse indexes.

shachain: clarify design in terms of binary tree, reverse indexes.

Olaoluwa Osuntokun came up with an alternative which used binary trees;
that's a much better way to explain it, so do that in design.txt and
update the implementation to work the same way.

Anthony Towns pointed out that the numbering is the reverse of the normal
hash chaining descriptions, so fix that too.

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