Simplify SubscriptionConnection

[ccbrown]

Previously SubscriptionConnection was a stream of GraphQLResponse<'a, S>, a newtype for:

Result<(Value<S>, Vec<ExecutionError<S>>), GraphQLError<'a>>

This doesn't really make much sense because subscription events cannot have any sort of pre-execution errors and the GraphQLResponse is always Ok. This is a bit misleading and complicates lifetimes as the output of the stream can otherwise be 'static.

This PR just makes SubscriptionConnection a stream of unwrapped tuples. This has the added benefit of allowing the user of the API to actually access the data / errors before serializing. Alternatively, it might make sense to create a new type for this, e.g. an ExecutionResult type.

This is something I came across while working on #709 that I figured could go in its own PR.

[LegNeato]

ExecutionResult sounds like a good idea. In general, I personally prefer to use explicit types rather than tuples where both sides would used a similar number of times...but it isn't a requirement.

[ccbrown]

Sounds good, will update shortly.

[ccbrown]

@LegNeato Updated. Went with ExecutionOutput since it's not a Result type and ExecutionResult already exists.

[LegNeato]

Looks good! Thanks! 🍻