Fixes #64.

Simplify validation by:

• refactoring the JSONSchema type, to avoid all the TypeScript issues that arised in v7
• auto-infering the returned type for basic types, so explicit casting is no longer needed

Includes breaking changes:

• For numbers:

  • deprecates JSONSchemaNumeric in favor of JSONSchemaNumber and JSONSchemaInteger

• For constants:

  • removes JSONSchemaConst
  • const is now part of JSONSchemaBoolean, JSONSchemaNumber, JSONSchemaInteger and JSONSchemaString
  • consequence: type is now required

• For enums:

  • removes JSONSchemaEnum
  • enum is now part of JSONSchemaNumber, JSONSchemaInteger and JSONSchemaString
  • consequence: type is now required
  • consequence: enums of different types are no longer possible (and it's better for consistency)

• For arrays:

  • type: 'array' is now required
  • items no longer accepts an array of JSON schemas
  • consequence: arrays with multiple types are no longer possible (and it's better for consistency, use an object if you mix types in a list)

• For objects:

  • type: 'object' is now required

• Removes JSONSchemaNull (makes no sense and can cause interoperability issues)

• Unsupported properties of the JSON schema standard are no longer accepted.