Change link() to shallowCopy() (issue #1343)

Instead of storing a pointer, the function copies the `VariantData`.

Benefits:
* smaller code
* no impact on programs that don't use this feature

Drawbacks:
* changes to the original variant are not always reflected on the copy
* modifying the original from the shallow copy leads to UB

src/ArduinoJson/Variant/VariantContent.hpp

@@ -31,8 +31,6 @@ enum {
   VALUE_IS_SIGNED_INTEGER = 0x0A,
   VALUE_IS_FLOAT = 0x0C,
 
-  VALUE_IS_POINTER = 0x10,
-
   COLLECTION_MASK = 0x60,
   VALUE_IS_OBJECT = 0x20,
   VALUE_IS_ARRAY = 0x40,
@@ -51,7 +49,6 @@ union VariantContent {
   UInt asUnsignedInteger;
   Integer asSignedInteger;
   CollectionData asCollection;
-  const class VariantData *asPointer;
   struct {
     const char *data;
     size_t size;