remove `DeliveryScheme`

I’m raising this issue now, since we’re talking about moving the parser package into its own library, and DeliveryScheme is part of that package. But the presence of this class throughout the Sangria code has been an annoyance.

Apparently Oleg got the idea from the Parboiled2 project. It was one of his earliest commits, and I suspect that it was a premature design choice. The original idea in Parboiled2 seems to be of dubious value itself.

So my suggestion is that, in the name of simplifying the code, that we return a Try where needed. This can easily be turned into an Either or a thrown exception.

[performantdata]

Code like this isn’t especially clear in the purpose of scheme:

def parse(input: String, config: ParserConfig = ParserConfig.default)(implicit
    scheme: DeliveryScheme[ast.Document]): scheme.Result =
  parse(ParserInput(input), config)(scheme)

when all it really means is

def parse(input: String, config: ParserConfig = ParserConfig.default): Try[ast.Document] =
  parse(ParserInput(input), config)(scheme)

It also makes code like this more opaque than need be:

case Success(res) if res.values.nonEmpty => scheme.success(res.values.head)
case Success(res) =>
  scheme.failure(
    new IllegalArgumentException("Input document does not contain any value definitions."))
case Failure(e: ParseError) => scheme.failure(SyntaxError(parser, input, e))
case Failure(e) => scheme.failure(e)

[performantdata]

OK let’s go for it.

I think you have everything you need already. I’m going to base the next PRs, for #466, on these.