Commands that are janet functions don't necessarily need argument string conversion

[andrewchambers]

It may be desirable to explicitly allow and write tests for things like:

($ (fn [table] (pp table)) @{"foo" "bar"})
@{"foo" "bar"}

Because a function is not an external command, but it still run after fork(), it could take arguments as other data types.

The counter argument is that it is consistent and simpler to just treat all commands as functions taking a list of strings. We need to flatten args anyway to deal with globbing.