[chain] Make `LocalChain::apply_changeset` infallible

[ValuedMammal]

It seems like the only way this can fail is if we don't find a base AND extension doesn't include block 0, but I think one of these will have to be true, i.e. extension must have a genesis block or else we have a base.

https://github.com/bitcoindevkit/bdk/blob/775e4aee3a9ec2d99adb129f7a9367a3a15c7032/crates/chain/src/local_chain.rs#L23-L30

https://github.com/bitcoindevkit/bdk/blob/775e4aee3a9ec2d99adb129f7a9367a3a15c7032/crates/chain/src/local_chain.rs#L43-L48

[ValuedMammal]

Perhaps more importantly, this function actually allows you to change the genesis block if the changeset happens to include it (although why would it?) which seems incongruent with most other methods

[notmandatory]

Is this something you think should be done for the 1.0.0-beta milestone? would it help make any wallet user facing functions infallible ?

[ValuedMammal]

I don't think it's terribly urgent and seems like it mostly affects developers who are working with LocalChain. The issue was originally motivated by a discussion with @rustaceanrob who pointed out that MissingGenesisError is surprisingly common for something that should be an inherent property of a LocalChain object.

[rustaceanrob]

What I mean by inherent or an invariant of LocalChain is if all constructors of LocalChain require a genesis block then calls of merging chains may only be fallible in that they do not connect