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

Resolves #584

* 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

Fix detection of IDependencyMap impl

A null conditional operator was required at line 70 to avoid a nullref.

Not pretty, but it works.

Conflicts:
	docs/guides/samples/dependency_module.cs
	src/Discord.Net.Commands/Utilities/ReflectionUtils.cs

Enhance Dependency Injection

Typo

Fix ParameterInfo.Type sometimes being null

never overestimate the end user

this should be handled by #520

Ensure aliases are built in the correct order

Replace ConcurrentBag<T> with HashSet<T>