[IoVM] asJson problems

[superbobry]

asJson isn't defined for nil, false and true singletons
Io> true asJson

  Exception: true does not respond to 'asJson'
  ---------
  true asJson                          Command Line 

Io> false asJson

  Exception: false does not respond to 'asJson'
  ---------
  false asJson                         Command Line 

Io> nil asJson

  Exception: nil does not respond to 'asJson'
  ---------
  nil asJson                           Command Line 

I suggest moving all the JSON related functionality from core to the JSON addon,
since it really isn't needed all the time.

[stevedekorte]

Fixed. Added asJson to the true, false, and nil singletons in libs/iovm/io/Object.io:

• true asJson → "true"
• false asJson → "false"
• nil asJson → "null" (JSON uses null, not nil)

Commits:

• wasm branch: 5f77a35
• master branch: 184ecb2

Closing.