Add Symbol Support

[nrdobie]

This would add support for Symbols for both keys and values.

Proposed usage:

import SJSON from '@onedeadpixel/superjson'
import { CriticalSymbol } from 'some-package' 

// In application setup
SJSON.symbols.add(CriticalSymbol)

const data = {
  [CriticalSymbol]: 'data'
}

const jsonString = SJSON.stringify(data) // => {
//   "__sj_symbol[[CritialSymbol]]": "data"
// }

// JSON string sent to another memory instance

const data = SJSON.parse(jsonString) // => {
//   [CriticalSymbol]: 'data'
// }

Possible Issues:

• Would rely on the symbols description to reverse it so two symbols with the same description could cause problems. Solution would be allow for description override, e.g. SJSON.symbols.add(CriticalSymbol, 'sp.CriticalSymbol')