Transformation for parameter dependent contexts

Transformation for parameter dependent contexts

Summary:
This diff adds rewriting for $x::C contexts.

```
function f(A $x)[$x::Co]: void {}

==>

function f<
  T$x as A,
  T$x@Co,
>(
  T$x $x
)[$x::Co]: void where
  T$x@Co = T$x::Co
{}
```

The transformation also handles the case of ?A, and when $x's type is already a type parameter. The latter is useful because the runtime will enforce $x as an A (upper bound enforcement) in this function

```
function f<T as A>(T $x): void {}
```

but it will not do so in the following case

```
function f<T as A, T$x as T>(T$x $x): void {}
```

The use of Rust here is a tradeoff. Pattern matching is very poor which makes this less readable than if the rewrite happened in Naming, but mutating the data structures in place as opposed to rebuilding them in a functional world ends up being less cumbersome overall.

Reviewed By: losvald

Differential Revision: D25608862

fbshipit-source-id: 3e14bf3825f8a1cd6f83d0511b7f641c0de89eb9