WIP: Add initial Set implementation

[ryanleary]

Description:
This PR starts working toward support for a Set method, enabling end users to use jsonparser to generate json efficiently.

Benchmark before change:

BenchmarkJsonParserLarge-8                    	  100000	     64362 ns/op	       0 B/op	       0 allocs/op
BenchmarkJsonParserMedium-8                   	  500000	     11447 ns/op	       0 B/op	       0 allocs/op
BenchmarkJsonParserEachKeyManualMedium-8      	 1000000	      6699 ns/op	     112 B/op	       2 allocs/op
BenchmarkJsonParserEachKeyStructMedium-8      	 1000000	      7142 ns/op	     704 B/op	      13 allocs/op
BenchmarkJsonParserObjectEachStructMedium-8   	  500000	     11236 ns/op	     656 B/op	      12 allocs/op
BenchmarkJsonParserSmall-8                    	10000000	       993 ns/op	       0 B/op	       0 allocs/op
BenchmarkJsonParserEachKeyManualSmall-8       	10000000	       663 ns/op	      80 B/op	       2 allocs/op
BenchmarkJsonParserEachKeyStructSmall-8       	10000000	      1014 ns/op	     256 B/op	       9 allocs/op
BenchmarkJsonParserObjectEachStructSmall-8    	10000000	      1098 ns/op	     240 B/op	       8 allocs/op

Benchmark after change:

BenchmarkJsonParserLarge-8                    	  100000	     66026 ns/op	       0 B/op	       0 allocs/op
BenchmarkJsonParserMedium-8                   	 1000000	     10733 ns/op	       0 B/op	       0 allocs/op
BenchmarkJsonParserEachKeyManualMedium-8      	 1000000	      6900 ns/op	     112 B/op	       2 allocs/op
BenchmarkJsonParserEachKeyStructMedium-8      	 1000000	      8238 ns/op	     704 B/op	      13 allocs/op
BenchmarkJsonParserObjectEachStructMedium-8   	  500000	     12796 ns/op	     656 B/op	      12 allocs/op
BenchmarkJsonParserSmall-8                    	10000000	      1041 ns/op	       0 B/op	       0 allocs/op
BenchmarkJsonParserEachKeyManualSmall-8       	10000000	       752 ns/op	      80 B/op	       2 allocs/op
BenchmarkJsonParserEachKeyStructSmall-8       	10000000	      1058 ns/op	     256 B/op	       9 allocs/op
BenchmarkJsonParserObjectEachStructSmall-8    	10000000	       975 ns/op	     240 B/op	       8 allocs/op

[ryanleary]

This still has some rough edges, but the spirit of it is right, I think. @buger is this something you'd be interested in merging when complete?

[bruth]

I see this in the Get method too (and likely you are following the existing practice), but what is the reason for initializing a new empty byte slice as suppose to return nil? If an error occurs, it is more idiomatic to return nil so it is clearer in the code.

[bruth]

I ran the following edge cases:

// output: `{},"foo:null`
jsonparser.Set([]byte("{}"), []byte("null"), "foo")

// output: `{},"foo":{"bar":null}`
jsonparser.Set([]byte("{}"), []byte("null"), "foo", "bar")

[bruth]

A couple more:

// output: `,"foo":null`
jsonparser.Set([]byte(""), []byte("null"), "foo")

// output: `1.323,"foo":null`
jsonparser.Set([]byte("1.323"), []byte("null"), "foo")

[guillaumerose]

Just to mention, that this PR enables to modify a json in-place (preserving keys order).
I think this is the only golang library that do that ! Thanks a lot.

[ryanleary]

Closing this in favor of #102 which @JoshKCarroll has kindly opened to continue this effort.