Implement schema parser

[kylef]

The current schema parser (conversion to data structures) is incredibly simplistic. There is an implementation in the Swagger 2 adapter (https://github.com/apiaryio/fury-adapter-swagger/blob/cc772bbd1ba10fdc61de0e5c069a5ab5abfcefa5/src/schema.js) which can be used for reference, our parser in lib/parser/oas/parseSchema.js can be improve significantly. We should implement a parser supporting https://github.com/OAI/OpenAPI-Specification/blob/50c152549263cda0f05608d514ba78546b390d0e/versions/3.0.0.md#schemaObject. When this is implemented I'd break it up significantly, we can for example handle array, and then object etc all separately.

One major difference between the OAS 2 implementation is we should provide source map information in the data struture output.