EntityQuery refactor.

[moonheart08]

This makes EntityQuery usable in foreach loops, as a replacement for EntityQueryEnumerator<T> and AllEntityQueryEnumerator<T>.

Additionally, 2, 3, and 4 argument EntityQuery has been introduced.

you can finally

foreach (var ent in myQuery)
{
    // ...
}

and expect it to not only work but also be optimal (I verified that the generated IL is expectedly efficient, and does not allocate.)

This additionally starts work on introducing a "proper" query engine, with DynamicEntityQuery, which allows you to mark entries in the query as optional (i.e. allowed to not be present, but looked up regardless) or excluded (must not be present).