read tasm object from memory

[aszepieniec]

This PR introduces two new methods on trait TasmObject:

• decode_iter takes an Iterator over BFieldElements and decodes it as a Self.
• decode_from_memory takes a memory (as HashMap<BFieldElement,BFieldElement>) and address (as BFieldElement) and decodes the object located there.

To enable this interface extension:

• the derive macro is modified
• a new struct MemoryIter is introduced; it converts a memory and address pair into an iterator over BFieldElements.