`Sentence`-based `ExprLinter`s lack the caching support of `Chunk`-based ones.

[hippietrail]

In lint_group.rs we can add linters in three ways:

• Call add() manually.
• Call add_chunk_expr_linter().
• Use the insert_struct_rule macro. This calls add()
• Use the insert_expr_rule macro. This calls add_chunk_expr_linter()

Since add_chunk_expr_linter() specifically works only for the Chunk Unit and provides caching that ExprLinters use for efficieny, this leaves no way for an ExprLinter that uses Sentence to make use of caching.

At the moment there are no such linters but I'm exploring linters which look for semantic context by checking all words in the surrounding sentence context against a list of keywords.

This does work with add() but will not benefit from caching.

I briefly looked at adding it but it seems to be more involved than I expected, so filing this instead since I might move on to other problems.