Refactor Scan optimization

Instead of hardcoding specific types and skipping type assertions based
on that, only check if a destination is a (sql.Scanner) after a failed
AssignTo.

This is slightly slower in the non-decoder case and *very* slightly
faster in the decoder. However, this approach is cleaner and has the
potential for further optimizations.

pgtype.go

@@ -3,7 +3,6 @@ package pgtype
 import (
 	"database/sql"
 	"reflect"
-	"time"
 
 	errors "golang.org/x/xerrors"
 )
@@ -404,27 +403,6 @@ func (ci *ConnInfo) DeepCopy() *ConnInfo {
 }
 
 func (ci *ConnInfo) Scan(oid uint32, formatCode int16, buf []byte, dest interface{}) error {
-	isFastType := false
-	switch dest.(type) {
-	case *int16:
-		isFastType = true
-	case *int32:
-		isFastType = true
-	case *int64:
-		isFastType = true
-	case *float32:
-		isFastType = true
-	case *float64:
-		isFastType = true
-	case *string:
-		isFastType = true
-	case *time.Time:
-		isFastType = true
-	case *[]byte:
-		isFastType = true
-	}
-
-	if !isFastType {
 	switch formatCode {
 	case BinaryFormatCode:
 		if dest, ok := dest.(BinaryDecoder); ok {
@@ -437,7 +415,6 @@ func (ci *ConnInfo) Scan(oid uint32, formatCode int16, buf []byte, dest interfac
 	default:
 		return errors.Errorf("unknown format code: %v", formatCode)
 	}
-	}
 
 	if dt, ok := ci.DataTypeForOID(oid); ok {
 		switch formatCode {
@@ -461,7 +438,11 @@ func (ci *ConnInfo) Scan(oid uint32, formatCode int16, buf []byte, dest interfac
 			}
 		}
 
-		if !isFastType {
+		assignToErr := dt.Value.AssignTo(dest)
+		if assignToErr == nil {
+			return nil
+		}
+
 		if scanner, ok := dest.(sql.Scanner); ok {
 			sqlSrc, err := DatabaseSQLValue(ci, dt.Value)
 			if err != nil {
@@ -469,9 +450,8 @@ func (ci *ConnInfo) Scan(oid uint32, formatCode int16, buf []byte, dest interfac
 			}
 			return scanner.Scan(sqlSrc)
 		}
-		}
 
-		return dt.Value.AssignTo(dest)
+		return assignToErr
 	}
 
 	// We might be given a pointer to something that implements the decoder interface(s),