Helper function that removes escapes in lexical units

from https://github.com/goavki/streamparser/commit/a21b53a641e60c009e415d56cfc51b3bd0e5eb0c

> keep escapes, want to be able to reconstruct the input
TODO: if one wants to remove them, there should be a function from lu's to lu's that does that

Start by reviewing the referenced commit a21b53a641e60c009e415d56cfc51b3bd0e5eb0c and the library's lexical-unit representation and parsing entry points. Define the helper's input and output behavior for escaped lexical units, then verify that the resulting lexical units have escapes removed without changing unrelated content.

Written by the indexing model from the issue text.