Refactor schema generation to use `printSchema`

printSchema from graphql-js allows to print human redable schema from schema object.
Generating schema this way will allow us to support many different types out of the box - if schema contains enum - we do not need to handle it anymore.

The way it could work is:

Users give us input schema
Schema then is being parsed into Graphql AST objects
We use visitor to collect some data in order to build resolvers.
We add extra types and operations directly to the schema
We print schema instead of generating it from strings.