Replace uses of libs/json with encoding/json. (#7534)

Where possible, replace uses of the custom JSON library with the standard
library. The custom library treats interface and unnamed lteral types
differently, so this change avoids those even where it would probably be safe
to switch them.

types/part_set.go

@@ -2,6 +2,7 @@ package types
 
 import (
 	"bytes"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -10,7 +11,6 @@ import (
 	"github.com/tendermint/tendermint/crypto/merkle"
 	"github.com/tendermint/tendermint/libs/bits"
 	tmbytes "github.com/tendermint/tendermint/libs/bytes"
-	tmjson "github.com/tendermint/tendermint/libs/json"
 	tmmath "github.com/tendermint/tendermint/libs/math"
 	tmproto "github.com/tendermint/tendermint/proto/tendermint/types"
 )
@@ -365,7 +365,7 @@ func (ps *PartSet) MarshalJSON() ([]byte, error) {
 	ps.mtx.Lock()
 	defer ps.mtx.Unlock()
 
-	return tmjson.Marshal(struct {
+	return json.Marshal(struct {
 		CountTotal    string         `json:"count/total"`
 		PartsBitArray *bits.BitArray `json:"parts_bit_array"`
 	}{

types/vote_set.go

@@ -2,12 +2,12 @@ package types
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"strings"
 	"sync"
 
 	"github.com/tendermint/tendermint/libs/bits"
-	tmjson "github.com/tendermint/tendermint/libs/json"
 	tmproto "github.com/tendermint/tendermint/proto/tendermint/types"
 )
 
@@ -495,7 +495,7 @@ func (voteSet *VoteSet) StringIndented(indent string) string {
 func (voteSet *VoteSet) MarshalJSON() ([]byte, error) {
 	voteSet.mtx.Lock()
 	defer voteSet.mtx.Unlock()
-	return tmjson.Marshal(VoteSetJSON{
+	return json.Marshal(VoteSetJSON{
 		voteSet.voteStrings(),
 		voteSet.bitArrayString(),
 		voteSet.peerMaj23s,