Implement json.Marshal just for strings

[ashish-goswami]

We are using json.Marshal() in our code. This function is very generic function and can be used to
marshal any type. For our use case, most of the time, this is called for string type. When we dig deeper into the function's code, most of the time is spent on finding the right encoder function for type. In our case stringEncoder.
This PR takes away the code of encodeState.string() and defines internally, which is just called for
string type. Here are the benchmarks:

go test -v -run ^$ -bench BenchmarkJsonMarshal -benchtime=20s
[Decoder]: Using assembly version of decoder
goos: linux
goarch: amd64
pkg: github.com/dgraph-io/dgraph/query
BenchmarkJsonMarshal/STDJsonMarshal-largestring-16         	11113017	      2128 ns/op
BenchmarkJsonMarshal/stringJsonMarshal-largestring-16      	12233304	      1957 ns/op
BenchmarkJsonMarshal/STDJsonMarshal-smallstring-16         	100000000	       252 ns/op
BenchmarkJsonMarshal/stringJsonMarshal-smallstring-16      	271762213	        87.7 ns/op
BenchmarkJsonMarshal/STDJsonMarshal-specialchars-16        	37255737	       636 ns/op
BenchmarkJsonMarshal/stringJsonMarshal-specialchars-16     	52765609	       463 ns/op
PASS
ok  	github.com/dgraph-io/dgraph/query	159.218s

This change is 

[manishrjain]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ashish-goswami and @pawanrawal)

---

query/outputnode.go, line 232 at r1 (raw file):

	switch v.Tid {
	case types.StringID, types.DefaultID:
		return stringJsonMarshal(v.Value.(string)), nil

switch the type. Use stringJsonMarshal, otherwise json.Marshal.

---

query/table.go, line 7 at r1 (raw file):

import "unicode/utf8"

Can you mention which file did we copy and from where.

[pawanrawal]

Reviewed 1 of 3 files at r1, 2 of 2 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @ashish-goswami)