Add hackIgnoreRefs option

• #82: add hackIgnoreRefs option for preventing circular references

Support generation of Hack Enum

#81

Bugfix: Allow generating empty shapes

#80

Generate unions for shapes

• #78: Generate unions of shapes
• #77: CI: Use Docker
• #76: Generate Nonnull Types for Unions

Generate unions for oneOf schemas

• #74: Support type inference for oneOf schemas

Improvements to `discardAdditionalProperties`

• #73: Make discardAdditionalProperties an object-level concern

Improve generated type for anyOf constraints

• #67: Generate "Union" Types for anyOf Constraint. Supports inferring the lowest upper bound type for anyOf constraints. Does not yet handle shapes or dicts, but does handle arraykeys, vecs and nums. Introduces a Typing namespace as a foundation atop which to build more complex type inference.

Use null type; handle coercion for anyOf

• #68: Handle string to object coercion correctly for optimized anyOf constraints
• #69: Type NullBuilder correctly

Alias top-level refs

Instead of doing a no-op when the schema is a top-level ref, generate a validator and type which just alias the referenced validator and type. This fits better with existing tooling which expects a type and validator to be generated.

Support top-level refs; support uniqueItems when items contains a ref

• Support top-level refs (e.g., schemas declared as {"$ref": "..."}. For these schemas, we'll ensure the referenced schema has a generated validator and type, but we won't generate any new code.
• Correctly handle the case where an array specifies uniqueItems and has an item which is a $ref.
• Breaking change: Codegen::build() no longer returns a file. Call getFile() on the Codegen object to get the generated file.