Typecheck providers

Summary:
```
MyInfo = provider(fields = ["x"])

def test():
  MyInfo(x = 1).y
```

is compile time error now.

Reviewed By: ianlevesque

Differential Revision: D48933487

fbshipit-source-id: bac5f2c8f18aba1354994cf3a0e061936fc4dd7e

Author: stepancheg

starlark/src/typing/ty.rs

@@ -429,7 +429,7 @@ impl Ty {
     }
 
     /// Iterate over the types within a union, pretending the type is a singleton union if not a union.
-    pub(crate) fn iter_union(&self) -> &[TyBasic] {
+    pub fn iter_union(&self) -> &[TyBasic] {
         &self.alternatives
     }