Add with-vars

This helps for temporarily setting vars in a safe
manner that is guaranteed not to leave vars in a bad state
(assuming that a fiber does not emit debug or use signal and
 is never resumed).

test/suite7.janet

@@ -184,4 +184,10 @@
 
 (assert (= (string '()) (string [])) "empty bracket tuple literal")
 
+# with-vars
+(var abc 123)
+(assert (= 356 (with-vars [abc 456] (- abc 100))) "with-vars 1")
+(assert-error "with-vars 2" (with-vars [abc 456] (error :oops)))
+(assert (= abc 123) "with-vars 3")
+
 (end-suite)