This commit implements a newtype style custom derive
for scalars via `#[derive(GraphQLScalarValue)]`, which now
supports both deriving a base enum scalar type and newtypes.
For newtypes, the `#[graphql(transparent)]` attribute is
required.
This commit:
* implements the derive
* adds integration tests
* updates the book
* Use mdbook for building the book
* Update book config
* Update book hierarchy to work properly with mdbook
This necessitated adding place-holder index pages
since mdbook does not suppoert stand-alon menu items
* Update tests to use 2018 edition
* Fix various compilation errors in the tests