[150 SigmaUSD] Implement MultiplyGroup op for GroupElement.

[greenhat]

Create a new struct in a separate module(file) in mir module and add it to Expr enum.
The constructor should check the types of the arguments (see Fold::new, ByIndex::new, etc.).

Serialization

• Implement SigmaSerializable trait and add node's op code in serialization::expr;
• Implement Arbitrary trait and add serialization roundtrip PBT test for this node;

Evaluation

• Implement Evaluable trait in ergotree-interpreter crate;
• Add evaluation tests;

Check out existing IR node implementation (Fold, ByIndex) for details.
Check out the detailed implementation instructions for Append op in #301

Reference Scala version:
IR node and eval - https://github.com/ScorexFoundation/sigmastate-interpreter/blob/89afefa935d52dd83741f6530f97e4c935992f76/sigmastate/src/main/scala/sigmastate/trees.scala#L1087-L1106

[ApexTheory]

I have started on this issue.

[greenhat]

Done in #316.

[greenhat]

Check out your DMs in our discord, please. Your bounty is ready to be sent.

[greenhat]

Thank you! Bounty is sent.