Add huffman tree, by block number:

prooflen: proof path 9-248(68.92+/-52), hashes 89-1513(474.08+/-3.4e+02)

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

Reduce size_t to int; we don't need > 4 billion, and we can no longer free
when we have a target.

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