Newly created type variables get exactly the jkind from the annotation

[dkalinichenko-js]

Change type annotations on new variables from being upper bounds to being exact values. This makes examples like val f : ('a : value_or_null) -> 'a check 'a : value_or_null properly instead of confusingly inferring 'a : value.

Unfortunately, this makes typechecking dependent on the order annotations: val f2 : ('a : value_or_null) -> 'a -> 'a infers 'a : value_or_null, but val f2' : 'a -> ('a : value_or_null) -> 'a infers 'a : value. We can probably fix this too, by defaulting only at the end when we know there are no annotations on 'a.

It also would be nice to require equality instead of subkinding here. Perhaps it's not OCaml-y, but it's way less confusing.

[goldfirere]

As discussed on Slack, I think the order-dependence here is pretty bad. I'd like to search for a different solution.