implemented 'injectStmt'; more debug support

doc/nimrodc.txt

@@ -468,6 +468,19 @@ proc is declared in the generated code:
 
   proc myinterrupt() {.codegenDecl: "__interrupt $# $#$#".} =
     echo "realistic interrupt handler"
+
+
+InjectStmt pragma
+-----------------
+
+The `injectStmt`:idx: pragma can be used to inject a statement before every
+other statement in the current module. It is only supposed to be used for
+debugging:
+
+.. code-block:: nimrod
+  {.injectStmt: gcInvariants().}
+  
+  # ... complex code here that produces crashes ...
 
 
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