pgtype/hstore: Save 2 allocs in database/sql Scan implementation

Remove unneeded string to []byte to string conversion, which saves 2
allocs and should make Hstore text scanning slightly faster.

The Hstore.Scan() function takes a string as input, converts it to
[]byte, and calls scanPlanTextAnyToHstoreScanner.Scan(). That
function converts []byte back to string and calls parseHstore. This
refactors scanPlanTextAnyToHstoreScanner.Scan into
scanPlanTextAnyToHstoreScanner.scanString so the database/sql Scan
function can call it directly, bypassing this conversion.

The added Benchmark shows this saves 2 allocs for longer strings, and
saves about 5% CPU overall on my M1 Pro. benchstat output:

goos: darwin
goarch: arm64
pkg: github.com/jackc/pgx/v5/pgtype
              │  orig.txt   │              new.txt               │
              │   sec/op    │   sec/op     vs base               │
HstoreScan-10   1.334µ ± 2%   1.257µ ± 2%  -5.77% (p=0.000 n=10)

              │   orig.txt   │               new.txt               │
              │     B/op     │     B/op      vs base               │
HstoreScan-10   2.094Ki ± 0%   1.969Ki ± 0%  -5.97% (p=0.000 n=10)

              │  orig.txt  │              new.txt              │
              │ allocs/op  │ allocs/op   vs base               │
HstoreScan-10   36.00 ± 0%   34.00 ± 0%  -5.56% (p=0.000 n=10)

pgtype/hstore.go

@@ -42,7 +42,7 @@ func (h *Hstore) Scan(src any) error {
 
 	switch src := src.(type) {
 	case string:
-		return scanPlanTextAnyToHstoreScanner{}.Scan([]byte(src), h)
+		return scanPlanTextAnyToHstoreScanner{}.scanString(src, h)
 	}
 
 	return fmt.Errorf("cannot scan %T", src)
@@ -236,14 +236,18 @@ func (scanPlanBinaryHstoreToHstoreScanner) Scan(src []byte, dst any) error {
 
 type scanPlanTextAnyToHstoreScanner struct{}
 
-func (scanPlanTextAnyToHstoreScanner) Scan(src []byte, dst any) error {
+func (s scanPlanTextAnyToHstoreScanner) Scan(src []byte, dst any) error {
 	scanner := (dst).(HstoreScanner)
 
 	if src == nil {
 		return scanner.ScanHstore(Hstore(nil))
 	}
+	return s.scanString(string(src), scanner)
+}
 
-	keys, values, err := parseHstore(string(src))
+// scanString does not return nil hstore values because string cannot be nil.
+func (scanPlanTextAnyToHstoreScanner) scanString(src string, scanner HstoreScanner) error {
+	keys, values, err := parseHstore(src)
 	if err != nil {
 		return err
 	}

pgtype/hstore_test.go

@@ -288,3 +288,22 @@ func BenchmarkHstoreEncode(b *testing.B) {
 		})
 	}
 }
+
+func BenchmarkHstoreScan(b *testing.B) {
+	strs := []string{
+		"",
+		`"a"=>"b"`,
+		`"a"=>"100", "b"=>"200", "c"=>"300", "d"=>"400", "e"=>"500"`,
+	}
+
+	var h pgtype.Hstore
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		for _, str := range strs {
+			err := h.Scan(str)
+			if err != nil {
+				b.Fatal(err)
+			}
+		}
+	}
+}