Typescript improvements and package size reduction

The primary purpose of this release is two fold:

1. Automatic model type inference (and overall better/more strict typescript definitions).
2. Simplified API (removed mapState callback support).

Note that this release introduces significant breaking API changes (hence the 2.0 release).

• Breaking: useProvider API changes (see: useProvider)
• Breaking: useCreateStore API changes (see: useCreateStore)
• Breaking: useStore API changes (see: useStore)

A more in depth explanation of the typescript changes (the primary driver of these API changes) can be found on the mobx-store-provider typescript documentation.