Skip (or error) on duplicate keys

[planetis-m]

In JSON either action is valid for duplicate keys. Currently we process every duplicate key with a potential performance penalty. A better alternative would be to generate code like:

var
  onceBanana = false
eat(p, tkCurlyLe)
while p.tok != tkCurlyRi:
  if p.tok != tkString:
    raiseParseErr(p, "string literal as key")
  case p.a
  of "banana":
    discard getTok(p)
    eat(p, tkColon)
    if not onceBanana:
      onceBanana = true
      initFromJson(dst.banana, p)
    else:
      when defined(emiDuplicateKey):
        skipJson(p)
      else: raiseParseErr(p, "duplicate object field")
  else:
    raiseParseErr(p, "valid object field")
  if p.tok != tkComma:
    break
  discard getTok(p)
eat(p, tkCurlyRi)

...and leave it up to the user to specify whether it should error or skip them.