Squashed commit of the following:
Squashed commit of the following:

commit d9b99f5ccd320bcfc42c90626141d68614aff3b9
Author: Neil Alexander <neilalexander@users.noreply.github.com>
Date:   Mon Jun 1 17:26:49 2020 +0100

    Remove unnecessary type

commit 504ef926060e120ba054304e285060669de11315
Author: Neil Alexander <neilalexander@users.noreply.github.com>
Date:   Thu May 28 18:19:22 2020 +0100

    Define more send-to-device structs

commit 0aa540ad74d1fdbbdbd4506ce809a6449a3ab390
Author: Neil Alexander <neilalexander@users.noreply.github.com>
Date:   Thu May 28 14:14:45 2020 +0100

    Add SendToDeviceEvent

matrix-org/gomatrixserverlib,matrix-org/gomatrixserverlib