Only allow channel-ids of the form channel-{N}

[hu55a1n1]

Summary of Bug

The ChannelId::FromStr impl does check if the given string is a valid ICS24 identifier but doesn't check if it is of the form channel-{N}.

Version

master

Acceptance Criteria

• It shouldn't be possible to create a channel that isn't of the form channel-{N}.
• Should we remodel ChannelId as ChannelId(u64) instead?

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[romac]

Good catch! Let's model it as a newtype of u64 indeed!