MSC: Replace event IDs with hashes

[erikjohnston]

Documentation: https://github.com/matrix-org/matrix-doc/blob/erikj/event_id_hashes/proposals/1659-event-id-as-hashes.md

This would basically mean instead of having IDs looking like $foobar:example.com they'd be something like: 7EQORdPaApfus7VgUPyupQ9d7P574HbgqqNKzgY7RoE.

This would then mean that hashes wouldn't need to be included when referencing events in e.g. prev_events.

Open questions include: how do we specify the type of hash? Or do we mandate e.g. sha256 and if we want to change we create a new room version?

A fuller writeup will be along shortly. see PR #1659

[ara4n]

Related: #1127

[erikjohnston]

(MSC has been relabelled to MSC1659)