Feature: flatten root BlockState for SemanticStackContext

[mrLSD]

Description

The current BlockState is represented as a tree with one parent and many children. It represents blocks as a tree with branches and leaves. Each block state has a context that contains SemanticStack. It means that all contexts also represented as a tree.

There is an unknown entry point to the branch or leaf of context. And it's a bit tricky to flatten root context for endpoint Codegen. To simplify the fetching process for Codegen backends, there is a proposal:

• for the parent BlockState "flatten" all children's contexts. It means parent context will also include children's contexts. And it has linear representations. At the same time, it will exist children BlockState with their own context
• root BlockState will contain flattened linear context.