It's impossible to add non-reference types to the map, so why allow pulling them out of it.

Resolves #584

Allows null embeds to be passed into ModifyAsync

* Improve parameter precondition type safety

Also removes some terrible code which was left over when I first
implemented parameter preconditions. I don't know why that was there.

With this commit, parameter preconditions should be much safer as they
use generic methods instead of janky casting of objects.

* Remove generic CheckPreconditions method

* Removed GuildUserExtensions and moved the methods to IGuildUser and implementations

* Made changes per fox's suggestion: Change->Modify. New Modify overload.

* Oops

* Per Volt: reimplemented new endpoints

* Fixing broken docstrings

* I forgot that docstrings are XML

* Implemented atomic add/remove role endpoints

* Removed  so people aren't irked

* Added single-item role add/remove methods

Fix detection of IDependencyMap impl