Implement object modification instructions

[marekjm]

Implement special instructions for modifying attributes stored in objects. This issue is related to #128 .

There are two instructions suggested:

• insert: insert an attribute into an object (possibly overwriting old value stored under the same key),
• remove: remove an attribute from an object and place it inside a register,

These two instructions have move semantics, so no copying is performed by either of them.