[250 SigmaUSD] Implement Option.map, filter methods

[greenhat]

Create ergotree_ir::types::soption module, add STypeCompanion and SMethod instances (see sgroup_elem module for examples).
Add EvalFn in ertotree-interpreter::eval::soption and include it in ergotree-interpreter::eval::smethod_eval_fn

See existing method implementations in sgroup_elem, scoll and sbox for examples;

Reference Scala version:
https://github.com/ScorexFoundation/sigmastate-interpreter/blob/0c5c5b515ccb452536e440469b56d763d5ba30ec/sigmastate/src/main/scala/sigmastate/types.scala#L1283-L1298

[ApexTheory]

I have started on this issue.

PR open https://github.com/ergoplatform/sigma-rust/pull/388/files

[greenhat]

Done in #388.

[greenhat]

Bounty is sent.