Introduce `(PortId, ChannelId)` domain type

[adizere]

Crate

ibc

Summary

Since we started working on ICS04 handlers(#452) we noticed that there is a recurring need to encapsulate the tuple (PortId, ChannelId) as a type. This tuple is needed in error types:

ibc-rs/modules/src/application/ics20_fungible_token_transfer/error.rs

Lines 19 to 20 in d625001

	#[error("Missing channel for port_id {0} and channel_id {1} ")]
	ChannelNotFound(PortId, ChannelId),

or as parameters to structures:

https://github.com/informalsystems/ibc-rs/blob/71d9baa6a2eed5a9286f9fe88f1cfacbd51e19d8/modules/src/mock/context.rs#L86-L86

or as parameters to methods:

https://github.com/informalsystems/ibc-rs/blob/71d9baa6a2eed5a9286f9fe88f1cfacbd51e19d8/modules/src/mock/context.rs#L279-L282

So there's ample reasons to introduce a new type for capturing the two.

Proposal

The new type should probably reside in ICS04. Potential names:

pub struct PortChannel(PortId, ChannelId);
pub struct BoundChannel(PortId, ChannelId);
pub struct AuthorizedChannel(PortId, ChannelId);

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[romac]

How about IbcSocket? Much like a TCP socket consists of an IP address and a port number.

[adizere]

How about IbcSocket? Much like a TCP socket consists of an IP address and a port number.

I love this name.

I would propose the following plan:

1. prepare a PR and motivation for introducing the name IbcSocket (or just Socket) upstream in the cosmos/IBC spec repo, and then subsequently in the .proto files (SDK and IBC-go). The changes should be mostly aesthetic and primarily in ICS 004.
2. adopt the name in our own repo.

We can also consider moving faster and not waiting on the IBC spec team, but that would mean we'd need to do two sets of changes (once for the initial ibc-rs-specific changes, and a second time to adopt the .proto files once modifications flow from upstream into our repo).