dhall-lang/dhall-haskell

This removes the need for `R2`

Maps unnamed data fields to the positional record keys. For example:

```haskell
data Foo = Foo Bool Integer Natural deriving (Generic, Interpret)
```

... is mapped to this Dhall data type:

```haskell
Record { _1 : Bool, _2 : Integer, _3 : Natural }
```

Add `Interpret`/`Inject` instances for `[]`

Uses R2 intermediate type to represent tuples in Dhall.

Note: This is a breaking change because this changes the type of `loadWith`

It is convenient to check equality of Expr Src X sometimes.

Adds trivial comments on extend, extract functions.

Fixes #26.