feature: Improved typescript types

The TypeScript typings of Immer have been rewritten, and support better inference now and are more accurate. The best practices are documented here: https://immerjs.github.io/immer/typescript. Please visit that page first when running into any compile errors after updating. 

The following changes have been made:

Immer can now infer the type of the `draft` for curried producers, if they are directly passed to another function / context from which the type can be inferred. So lines like these will now be strongly typed: `setTodo(produce(draft => { draft.done = true }))`, where `setTodo` is a React state updater (for example). Fixes #720.

Immer will now better respect the original types passed to a producer, and generally not wrap any returned types with `Immutable` automatically.

BREAKING CHANGE: It is no longer allowed to return `nothing` from a recipe if the target state doesn't accept `undefined`.

BREAKING CHANGE: It is no longer allowed to return arbitrary things from a recipe. Recipes should either return nothing, or something that is assignable to the original state type. This will catch mistakes with accidental returns earlier.