[TM-374] Removing `NStar`

Description

Currently Notes are represented as either NStar or actual notes. The need to handle special ctor makes code too complicated at places and prevents some features from being implemented.

This optimization has been introduced to avoid O(n) types comparison (which happens, potentially, in typechecking of every single instruction).

Related issue(s)

https://issues.serokell.io/issue/TM-374

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

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• My code complies with the style guide.