Protect the Args field of NAryExpr when it is immutable.

* Made changing the reference throw an exception if the NAryExpr was
constructed as immutable
* Changed the type of NAryExpr.Args to be IList<Expr> rather than
List<Expr> so that when the NAryExpr is immutable I can return
``_Args.AsReadOnly()`` (instead of ``_Args``) which returns a read
only wrapper around the List<Expr> so that clients cannot change the
list.

I came across two problems

* Making this change required changing types all over the place (from
List<Expr> to IList<Expr>). I feel that changes are extensive enough
that there's a good chance that out of tree clients using Boogie's
libraries might break. I've waited for a code review but this didn't
happen so I'm committing anyway.

* I came across something that looks like bad design of the IAppliable
interface which potentially breaks immutability enforcement. I've left
this as a "FIXME" in this. Here's the problematic method.

```
Type Typecheck(ref List<Expr>/*!*/ args, out
TypeParamInstantiation/*!*/ tpInstantiation, TypecheckingContext/*!*/
tc);
```

It potentially allows the instance of the args to be changed which
seems very suspect.

1 files changed, 1 insertions, 1 deletions

diff --git a/Source/Core/Duplicator.cs b/Source/Core/Duplicator.cs
index 4350571a..becd1791 100644
--- a/Source/Core/Duplicator.cs
+++ b/Source/Core/Duplicator.cs
@@ -214,7 +214,7 @@ namespace Microsoft.Boogie {
       Contract.Ensures(Contract.Result<Expr>() != null);

       return base.VisitExpr((Expr)node.Clone());

     }

-    public override List<Expr> VisitExprSeq(List<Expr> list) {

+    public override IList<Expr> VisitExprSeq(IList<Expr> list) {

       //Contract.Requires(list != null);

       Contract.Ensures(Contract.Result<List<Expr>>() != null);

       return base.VisitExprSeq(new List<Expr>(list));