Additional scan performance improvements

Move typeStack to ScanContext, so it is shared between rows.Scan calls.
Use string.Builder for string concatenations.
Simplify value assign logic.
Move convert value to the last assign step (needs for type conversions are rare).

tests/postgres/with_test.go

@@ -2,7 +2,6 @@ package postgres
 
 import (
 	"context"
-	"fmt"
 	"github.com/go-jet/jet/v2/internal/testutils"
 	. "github.com/go-jet/jet/v2/postgres"
 	"github.com/go-jet/jet/v2/tests/.gentestdata/jetdb/northwind/model"
@@ -864,5 +863,4 @@ WHERE orders1."orders.order_id" < $1;
 	err := stmt.Query(db, &dest)
 	require.NoError(t, err)
 	require.Len(t, dest, 72)
-	fmt.Println(len(dest))
 }