drnim: tiny progress (#13882)

* drnim: tiny progress
* refactoring complete
* drnim: prove .ensures annotations
* Moved code around to avoid code duplication
* drnim: first implementation of the 'old' property
* drnim: be precise about the assignment statement
* first implementation of --assumeUnique
* progress on forall/exists handling

drnim/tests/tbasic_array_index.nim

@@ -38,6 +38,14 @@ proc xu(a: uint) =
     let chunk = range[1u32..10u32](a)
     ru chunk
 
+proc parse(s: string) =
+  var i = 0
+
+  while i < s.len and s[i] != 'a':
+    inc i
+
+parse("abc")
+
 {.pop.}
 
 p([1, 2, 3], [4, 5])