[RFC] Add Validation rule for unique directives per location.

[leebyron]

This rule was suggested by @jjergus (#223) to remove ambiguity from execution where the @skip and @ignore directive rules make the assumption that only one is defined per selection.

In general I think this assumption should be upheld by valid queries overall, not just for @skip and @include so that it can be relied upon for any usage of directives. For example, this allows a potential optimization to use a hashmap keyed on directive name to represent the directives at a location rather than an array.

Closes #223

[stubailo]

I guess this means that certain directives will need to include array arguments, rather than having one argument and using the directive multiple times, eg:

{
  comments @refetchOn(channels: ["newComment", "removedComment"])
}

Rather than:

{
  comments @refetchOn(channel: "newComment") @refetchOn(channel: "removedComment")
}

Seems reasonable, to require the directive to decide if it wants to accept multiple inputs.

[leebyron]

Exactly. I think the latter API is more explicit to the possible behavior.

[jjergus]

A particularly interesting example is that this is also valid:

fieldA @skip(if: $foo)
fieldA @skip(if: $bar)

[leebyron]

Great point - I should add that

[leebyron]

Further rationale after an offline discussion with @dschafer:

While this is explicitly a limiting change and therefore potentially breaking, by reducing what is possible now it increases future option value. This change is one that we could always change back in some capacity in the future in a non-breaking way should an explicit need arise. Barring any such need today, it's in our best interest to preserve that option-value.