Rewrite permission checks

[GnomedDev]

Based on #286, but as a straight up rewrite instead of rework.

This splits permission checks up into all possible branches, and performs them efficiently without unnecessary cache or http calls. I have also been liberal with panicking on incorrect assumptions, as that's what they are for.

The algorithm described is written below:

for interactions (which is infallible):

• author permissions: interaction.member.permissions (if member is none, use Permissions::dm_permissions, expect permissions)
• bot permissions: interaction.app_permissions (expect, since it is documented as always Some)

for prefix (only fallible with no cache):

• author permissions:
• • cache: message.author_permissions()
• • no cache: PartialGuild::user_permissions_in, Message::member
• bot permissions:
• • cache: Guild::user_permissions_in, bot member is always cached
• • no cache: PartialGuild::user_permissions_in, PartialGuild::member