Performance improvments

[vektah]

Added a benchmark for a simple query and make a 2 quick optimisations:

• don't use strconv.Quote when writing object keys to save a bunch of allocs
• execute the first concurrent field on the request goroutine, instead always creating a new goroutine.

before

    go test -benchtime=5s -bench=. -benchmem
    goos: linux
    goarch: amd64
    pkg: github.com/99designs/gqlgen/example/starwars
    BenchmarkSimpleQueryNoArgs-8      200000             32680 ns/op            6357 B/op        126 allocs/op
    PASS
    ok      github.com/99designs/gqlgen/example/starwars    9.901s

You can see morestack dominating the graph here (28%). This happens because we create new goroutines around every resolver, and even though go does recycle old goroutine in a pool, it doesn't maintain their stacks.

after

    go test -benchtime=5s -bench=. -benchmem    
    goos: linux
    goarch: amd64
    pkg: github.com/99designs/gqlgen/example/starwars
    BenchmarkSimpleQueryNoArgs-8      300000             25093 ns/op            6453 B/op        114 allocs/op
    PASS
    ok      github.com/99designs/gqlgen/example/starwars    10.807s

~ 30% faster.

Ideas for future tweaks:

• 15% is still spent on morestack + 13% in starwars.ec_Human_starship: Marshalling arrays in results also happens concurrently, theres another high fanout go func() in here. This causes lots of short lived goroutines, which allocate more stack. Probably the next target.
• 8% in json.Marshal: This can be replaced with our internal json writer, avoiding all of the reflection and many allocations. Needs a little bit of thought around encoding extensions, do we fall back to json.Marshal?
• 7% in json.Decode: Maybe there is a faster json decoder out there for this?