Method type inference should only infer speakable types

```C#
void M<T>(T x)
{
  if (x == null) throw null;
  var y = Identity(x); 
  // currently we infer Identity<T!> (which you could not write explicitly), but should be Identity<T>
}
T Identity<T>(T z) => z;
```

The change should include inference of `var` and best type (`new [] { x }`). Those inferred types should be speakable too.

From LDM discussion 2018-05-12
FYI @AlekseyTs @cston 