dillon1000/react

Manually revert #2127 (allow mutating context in callbacks)

#2127 introduced a special type for the result of `useContext()` that was sort 
of ref-like. The intent was to allow code like this: 

``` 

function Foo() { 

const cx = useContext(...); 

function onEvent() { 

cx.foo = true; 

}; 

return <Bar onEvent={onEvent} />; 

} 

``` 

However, that code actually is allowed by the compiler by default. It's only a 
bailout when `@validateFrozenLambdas` is enabled. The "fix" in #2127 therefore 
wasn't strictly necessary to unblock rollout, and it's also flawed in a few 
ways: 

* First, `useContext(FooContext)` should have equivalent behavior to a custom 
hooks which does the same thing, ie `function useFooContext() { return 
useContext(FooContext) }`. Specializing the type of useContext makes the 
behavior different. 

* Second, it meant that even readonly accesses of the context inside a callback 
marked the function as capturing, which in turn prevented those callbacks from 
being memoized. 

So i'm reverting this and we'll have to think a bit more about this case.