Allow dynamically setting output for printers

Some functions like print and debug/stacktrace print
to a file, usually stdout. This file can now be optionally set
via a dynamic variable.

test/suite6.janet

@@ -88,4 +88,12 @@
            ))
         "int64 typed arrays")
 
+# Dynamic bindings
+(setdyn :a 10)
+(assert (= 40 (with-dyns [:a 25 :b 15] (+ (dyn :a) (dyn :b)))) "dyn usage 1")
+(assert (= 10 (dyn :a)) "dyn usage 2")
+(assert (= nil (dyn :b)) "dyn usage 3")
+(setdyn :a 100)
+(assert (= 100 (dyn :a)) "dyn usage 4")
+
 (end-suite)