Add constants to ChainHash for each Network

[jkczyz]

ChainHash::using_genesis_block can't be made const because it uses a match expression, which is only valid in Rust 1.46. Add individual constants as a workaround so that ChainHash can be used in const contexts.

[apoelstra]

ACK b1d8516

[tcharding]

ACK b1d8516

[tcharding]

Thanks for the contribution man!

[Kixunil]

Oh, I didn't see this soon enough

can't be made const because it uses a match expression

Actually, it can!

let hashes = [GENESIS_BLOCK_HASH_BITCOIN, GENESIS_BLOCK_HASH_TESTNET, GENESIS_BLOCK_HASH_SIGNET, GENESIS_BLOCK_HASH_REGTEST];
ChainHash(hashes[network as usize])

I don't mind making them public, it's useful anywas, but having const fn too would be nice. :)

[jkczyz]

Yeah, I could put that together. Is there any concern that adding a new Network type won't cause a compilation error? Maybe there is a way to cause a test breakage? Looks like core::mem::variant_count could be useful but is unfortunately unstable.

[Kixunil]

We could write a dummy test that matches all variants returning () and put a comment in it saying to remember adding the value into the fn. However we don't have #[non_exhaustive] on Network now which means we can't extend it. It seems bad but maybe there's a reason?

[apoelstra]

No reason, just that Network long predates #[non_exhaustive].

[Kixunil]

I'm surprised we missed it before. I guess I will go through all of enums at some point.

[apoelstra]

Yeah, we should -- but I think so far we only went through error enums because those were easy targets for non_exhaustie.