hstore: Make binary parsing 2X faster

* use []string for value string pointers: one allocation instead of
  one per value.
* use one string for all key/value pairs, instead of one for each.

After this change, one Hstore will share two allocations: one string
and one []string. The disadvantage is that it cannot be deallocated
until all key/value pairs are unused. This means if an application
takes a single key or value from the Hstore and holds on to it, its
memory footprint will increase. I would guess this is an unlikely
problem, but it is possible.

The benchstat results from my M1 Max are below.

goos: darwin
goarch: arm64
pkg: github.com/jackc/pgx/v5/pgtype
                               │   orig.txt   │               new.txt               │
                               │    sec/op    │   sec/op     vs base                │
HstoreScan/databasesql.Scan-10    82.11µ ± 1%   82.66µ ± 2%        ~ (p=0.436 n=10)
HstoreScan/text-10                83.30µ ± 1%   84.24µ ± 3%        ~ (p=0.165 n=10)
HstoreScan/binary-10             15.987µ ± 2%   7.459µ ± 6%  -53.35% (p=0.000 n=10)
geomean                           47.82µ        37.31µ       -21.98%

                               │   orig.txt   │               new.txt               │
                               │     B/op     │     B/op      vs base               │
HstoreScan/databasesql.Scan-10   56.23Ki ± 0%   56.23Ki ± 0%       ~ (p=0.324 n=10)
HstoreScan/text-10               65.12Ki ± 0%   65.12Ki ± 0%       ~ (p=0.675 n=10)
HstoreScan/binary-10             21.09Ki ± 0%   20.73Ki ± 0%  -1.70% (p=0.000 n=10)
geomean                          42.58Ki        42.34Ki       -0.57%

                               │  orig.txt   │               new.txt                │
                               │  allocs/op  │ allocs/op   vs base                  │
HstoreScan/databasesql.Scan-10    744.0 ± 0%   744.0 ± 0%        ~ (p=1.000 n=10) ¹
HstoreScan/text-10                743.0 ± 0%   743.0 ± 0%        ~ (p=1.000 n=10) ¹
HstoreScan/binary-10             464.00 ± 0%   41.00 ± 0%  -91.16% (p=0.000 n=10)
geomean                           635.4        283.0       -55.46%
¹ all samples are equal

pgtype/hstore.go

@@ -183,47 +183,43 @@ func (scanPlanBinaryHstoreToHstoreScanner) Scan(src []byte, dst any) error {
 
 	rp := 0
 
-	if len(src[rp:]) < 4 {
+	const uint32Len = 4
+	if len(src[rp:]) < uint32Len {
 		return fmt.Errorf("hstore incomplete %v", src)
 	}
 	pairCount := int(int32(binary.BigEndian.Uint32(src[rp:])))
-	rp += 4
+	rp += uint32Len
 
 	hstore := make(Hstore, pairCount)
+	// one allocation for all strings, rather than one per string
+	valueStrings := make([]string, pairCount)
+	// one shared string for all key/value strings
+	keyValueString := string(src[rp:])
 
 	for i := 0; i < pairCount; i++ {
-		if len(src[rp:]) < 4 {
+		if len(src[rp:]) < uint32Len {
 			return fmt.Errorf("hstore incomplete %v", src)
 		}
 		keyLen := int(int32(binary.BigEndian.Uint32(src[rp:])))
-		rp += 4
+		rp += uint32Len
 
 		if len(src[rp:]) < keyLen {
 			return fmt.Errorf("hstore incomplete %v", src)
 		}
-		key := string(src[rp : rp+keyLen])
+		key := string(keyValueString[rp-uint32Len : rp-uint32Len+keyLen])
 		rp += keyLen
 
-		if len(src[rp:]) < 4 {
+		if len(src[rp:]) < uint32Len {
 			return fmt.Errorf("hstore incomplete %v", src)
 		}
 		valueLen := int(int32(binary.BigEndian.Uint32(src[rp:])))
 		rp += 4
 
-		var valueBuf []byte
 		if valueLen >= 0 {
-			valueBuf = src[rp : rp+valueLen]
+			valueStrings[i] = string(keyValueString[rp-uint32Len : rp-uint32Len+valueLen])
 			rp += valueLen
-		}
 
-		var value Text
+			hstore[key] = &valueStrings[i]
-		err := scanPlanTextAnyToTextScanner{}.Scan(valueBuf, &value)
-		if err != nil {
-			return err
-		}
-
-		if value.Valid {
-			hstore[key] = &value.String
 		} else {
 			hstore[key] = nil
 		}