rebased :)

Looks legit but I'd like someone more experienced with probabilities than myself to confirm this is a good idea.

@Kixunil basically this restricts our "fidelity" in expressing relative probabilities to maximally 4 billion to one, and introduces a possible "optimization glitch" when we have more than 4 billion branches all of maximal weight (well formedness still fine).

I think it's very unlikely that we need to express something as more than 4 billion times more likely very often -- i.e. if you think you might use a signing path once per second for lightning, you'd use the other path once every 126 years.

I don't think this patch is critical, but some folks may not like using u128s internally.

yeah to be clear i personally don't think this patch is needed, so it's really up to others if they think it's needed.

Merging, since it has two ACKs for some time already and is quite trivial

I would like a post-merge ack i think maybe from @sanket1729 and @apoelstra if possible.

This was a follow up because there was some dissent on my use of u128 in a previous PR.

I know I authored it but i am neutral on this so i don't think the usual 2 acks works since usualy 2 acks is really 3, including the author's.

yep, concept ACK