interp: fix creation of binary composite types (#1391)

* interp: fix creation of binary composite types

Use direct assignment instead of reflect.Value Set method to
initialize a binary composite type, as for non binary types.
It ensures that a new reflect.Value is stored in the frame
instead of modifying a possibly existing one, which can defeat
the purpose of initializing variables in a body loop.

While there, remove the need to have and use a mutex on types.

Fixes #1381.

* review: rework a bit the test

Co-authored-by: mpl <mathieu.lonjaret@gmail.com>

interp/type.go

@@ -7,7 +7,6 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
-	"sync"
 
 	"github.com/traefik/yaegi/internal/unsafe2"
 )
@@ -110,7 +109,6 @@ type structField struct {
 
 // itype defines the internal representation of types in the interpreter.
 type itype struct {
-	mu          *sync.Mutex
 	cat         tcat          // Type category
 	field       []structField // Array of struct fields if structT or interfaceT
 	key         *itype        // Type of key element if MapT or nil