31.x branch

[Sjors]

I'm updating to the latest master, before the v31 branch-off, in #9.

A good time to create a v31 branch here is when either:

1. Bitcoin Core v31.0 is released; or
2. The Bitcoin Core master branch merges a .capnp change that's not meant to be back ported to their 31.x branch

We could do it right after #9, but it's less work to wait for (1) in case one of the release candidates needs more changes.

[Sjors]

Here it is: https://github.com/2140-dev/bitcoin-capnp-types/tree/31.x

I used #21 for a CI dry run.