Memory optimizations: User struct

[jchristgit]

At present we store the following attributes on the user struct that are
technically not used since we do not really provide OAuth2 functionality at the
moment (and apart from that, Nostrum is a bit big to provide that, although I
think using it as a library instead of an application would be a great
improvement):

• email
• mfa_enabled
• verified

The discriminator field has been deprecated by Discord and - to my knowledge,
but we should verify this - we can remove it, as it is just 0 for
compatibility reasons now. That said, I do remember seeing user accounts with
discriminators still set, but I believe these to be accounts that haven't logged
in for a while. If we're unsure, we should probably keep it.

Finally, the public_flags field should just be an integer used as a bitset,
and we should provide a function either in the user or in the flags module to
parse said bitset into a struct. I am not aware of people really using this
field in the first place.

In an ideal world, we could also save memory by not including the bot field
with every user object, as it will be false in > 99.99% of cases. But likely,
we can not sanely do it because structs require all their fields to be present.

The goal is to make nostrum the most efficient library on the planet. The BEAM
and its engineering marvels are on our side.

[jb3]

Discriminator is still in use by bots, until that changes we should keep it for that purpose, once that changes we can drop it.

Public flags will be a huge saving so we should work on that.