change syntax for callable

anything that can be applied is now callable, this is the first step for
closure, partial application, and call-by-value

test/simple/main.air

@@ -41,4 +41,16 @@ increment : fn number => number = function x is
 is_even : fn number => bool = function x is
   if x % 2 then true else false
 
+; invoke a function by name
 bruh : number = increment(one)
+
+; invoke a lambda function
+bruh1 : number = (function x is x)(1)
+
+; invoke a calculation returns a function
+bruh2 : fn bool => number = function cond is
+  (if cond then
+    function x is x + 1
+  else
+    function x is x + 2
+  )(0)