Add null-based string to std

std/string/array2null.lb

@@ -0,0 +1,7 @@
+@ 0
+ args 0
+ size
+ #exec std/rev.lb
+ push 0
+ size
+ #exec std/rev.lb

std/string/null/print.lb

@@ -0,0 +1,9 @@
+@ 0
+ args 1
+ dup
+ jz 1
+ putc
+ jmp 0
+
+@ 1
+ quit