Add when-with and if-with

This is useful for reading from files.

test/suite7.janet

@@ -286,4 +286,17 @@
   (file/seek f :set 0)
   (assert (= (string (file/read f :all)) "foo\n") "temp files work"))
 
+(var counter 0)
+(when-with [x nil |$]
+           (++ counter))
+(when-with [x 10 |$]
+           (+= counter 10))
+
+(assert (= 10 counter) "when-with 1")
+
+(if-with [x nil |$] (++ counter) (+= counter 10))
+(if-with [x true |$] (+= counter 20) (+= counter 30))
+
+(assert (= 40 counter) "if-with 1")
+
 (end-suite)