[TM-280] Multiple entrypoints (part 5)

Description

Pre-final MR which implements the remaining features - support entrypoints in morley executable and SELF instruction. Also, this MR contains some small refactorings.

In the next, last MR I'm going to perform the remaining cleanup.

Related issue(s)

https://issues.serokell.io/issue/TM-280

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
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