Remove entities and events helpers

[davfsa]

In preparation for msgspec, we need to get rid of entity helpers like get_x or fetch_x, always ensuring that we have suitable alternatives.

dm_user

First case that comes to mind when thinking about this is messaging users. The flow usually goes:

1. Getting or fetching the dm channel
2. Sending a message to that user.

A dm_user function could merge those into a single call for the user. We can also leverage the cache to check for known dm channels that are created (create_dm_channel already does this) and avoid 2 API calls.