Add a resource fit predicate.

Not linked into any scheduler yet.
Now you make me think about it, in the end state we might see most fields filled in for most objects.  (I take back the "exceedingly rare".)  Nodes might omit _limit_, because it would be the same as _request_, but could well have the other parts;  pods will probably have them all; containers won't have _capacity_.

Up to you whether it's worth having special, largely-redundant, data structures for these cases; I wouldn't.