Restore bare fn package compatibility

[isomorphisms]

Stop reserving ASCII fn; it now follows the ordinary SYMBOL path again, while λ and ƒ remain function-constructor spellings.

The compatibility smoke:

• exercises a representative function(call, fn) { fn <- ... } fixture;
• verifies both λ and ƒ still construct functions;
• installs unmodified current CRAN rlang, testthat, tibble, and dplyr from source under IR;
• loads all four and runs IR syntax (←, ÷, ≟) inside upstream testthat.

The README now states that fn is an ordinary identifier and that fn(...) is an ordinary call, not constructor syntax.

This is stacked on #5 because the smoke test uses the preferred ≟ spelling introduced there. It removes the parser blocker for isomorphisms/r-koans#2.

Closes #12.