Remove `traverse` `action`s' explicit function reference passing

[amorphedstar]

Currently, a recursive function object traverse must be provided as an argument for actions whenever when is :traverse. https://github.com/coalton-lang/coalton/blob/main/src/codegen/traverse.lisp#L57-L61 The recursive object reference never changes over a traversal, so it could be nice to fix the reference to something not explicitly passed, using something like dynamic extent.

[stylewarning]

I would say we want to use a special variable (e.g., a (defvar *foo*)) which is bound for the dynamic extent of the call