Omit Symbol properties for anyOf validation schema

[sinclairzx81]

This PR updates the anyOf code generation pass to omit Symbol properties for the AJV validation schema. This is done via JSON.stringify() which will automatically omit Symbols (and other non-JSON applicable types). This replaces the previous approach that used NodeJS's util.inspect().

Fixes #271.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[mcollina]

Good work!