Add target, for SPV paths to other than the genesis.

Result of 20 runs of ./test-trees $((1024 * 1024)) $((1024 * 1024 - 150)):

array: proof hashes 135-897(346.4+/-2.1e+02)
optimal: proof hashes 33-119(62.4+/-24)
maaku: proof hashes 65-223(146.2+/-47)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Proper argument handling, and option to remove maaku tree.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Two new tree modes.

Results for 20 runs of 1M:

array: proof hashes 367-2863(1416.4+/-7.8e+02)
optimal: proof hashes 315-2143(1031.7+/-5.6e+02)
breadth-batch: proof hashes 307-2327(1106.4+/-6.1e+02)
array-batch: proof hashes 282-2248(1050.6+/-6e+02)

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