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/tsetlen_invalidates.nim

@@ -1,12 +1,15 @@
 discard """
-  nimout: '''
-tsetlen_invalidates.nim(15, 12) Warning: cannot prove: 0 <= len(a) + -1; counter example: a.len -> 0 [IndexCheck]
+  nimout: '''tsetlen_invalidates.nim(12, 10) Warning: BEGIN [User]
+tsetlen_invalidates.nim(18, 12) Warning: cannot prove: 0 <= len(a) + -1; counter example: a`1.len -> 0
+a.len -> 1 [IndexCheck]
+tsetlen_invalidates.nim(26, 10) Warning: END [User]
 '''
   cmd: "drnim $file"
   action: "compile"
 """
 
 {.push staticBoundChecks: defined(nimDrNim).}
+{.warning: "BEGIN".}
 
 proc p() =
   var a = newSeq[int](3)
@@ -20,3 +23,4 @@ proc p() =
 {.pop.}
 
 p()
+{.warning: "END".}