Allow expanders to be used as functions

[mikera]

Sometimes it would be nice to be able to pass an expander as a function.

Considerations:

• When interpreted as a function, would logically expand to the equivalent of (fn [a b c ...] <expanded form using symbols a,b,c, ...>)
• Would allow code like (map or list1 list2)
• Not clear how this interacts with contexts?