switch handle hook from positional arguments to named arguments

[Conduitry]

This resolves #958 by switching the handle hook from positional arguments to named arguments. It also updates the appropriate tests and documentation.

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[Conduitry]

get_hooks seemed to be an unnecessary indirection, and I've removed it while updating this.

[dummdidumm]

Is there any way to make this more obvious to existing users, like print an error telling you that the contract changed if someone upgrades and uses the old contract? That could be removed later on.

[Conduitry]

Maybe in dev mode we could check hooks.handle.length? I'm not sure where would be the best place and mechanism to surface this warning though.

[Conduitry]

@pngwn mentioned in chat that adding more specific errors or warnings around breaking changes in beta would be unmanageable. I don't disagree with this. In beta, I don't feel much guilt about changing the API like this if it's an improvement, and at this stage I also don't foresee that terribly many issues being opened which we would then have to close and say 'read the changelog'.