Clarify snippet format
Explain how include-snippets works
Note -> Important @ create-snippet
Now a snippet stores a list of lines as its content for performance
reason
Completely change format from storing strings as snippets content to
lines
Add 'processed to snippet data structure to determine if a snippet has
already been processed via include-snippets
Describe how include-snippets should work
Add tip about box and mutability
Implement update-snippet/boxed
Include include-snippets @ generate-src
Implement process-include-include
do-include-snippet is a better for process-include-snippet
Terminology updated: include directive
Update @ include directive @ terminology
Implement is-include-directive?
Correct is-include-directive?
Implement get-included-snippet-name
Implement get-snippet-by-name
Uncomment do-include-snippet to test
Add one test for do-include-snippet