Do not treat all type constructors as injective

Do not treat all type constructors as injective

Summary:
The type-checker currently treats all type constructors as if they were injective. For example, given
```
newtype N<+T> = int
```
it treats the constraints `N<T1> <: N<T2>` and `T1 <: T2` as equivalent, even though they are not. Simplifying the former to the latter is sound, but not complete.

However, we observe that when type-checking for example a call site, we can rely on such sound-but-incomplete simplifications. Otherwise, we wouldn't be able to type-check even simple examples like the following:
```

function foo<T>(N<T> $x) : void {}

function bar(N<string> $y) : void {
  foo($y);
}
```
that require `T` to be instantiated with `string`.

Therefore, we continue treating all type constructors as injective if completeness is not required, as indicated by the value of `require_completeness` in `Typing_subtype.subtype_env`.

Reviewed By: andrewjkennedy

Differential Revision: D36444677

fbshipit-source-id: ca743055835687feff68b1fe7d2c15c5d38ba3d4