Read and update files on-fly

Implement iterative file reader, that don't load full file data into the memory, but processes the file in a loop without or with a small buffer

The issue does not name a file, test, or entry point. Locate the current file-reading path in the Go tool and trace how it reads and updates files; done means processing files iteratively without loading the full contents into memory, with only a small buffer if needed.

Written by the indexing model from the issue text.