Add support of dynamic property and index assignments

[metoule]

This allows to write:

dynamic dyn = new ExpandoObject();
dyn.Foo = new int[5];

var interpreter = new Interpreter().SetVariable("dyn", (object)dyn);
interpreter.Eval("dyn.Foo[2] = 5");
interpreter.Eval("dyn.Bar = \"foo\"");

Assert.That(dyn.Foo[2], Is.EqualTo(5));
Assert.That(dyn.Bar, Is.EqualTo("foo"));

The fix is bit tricky: when we parse the assignment, the left operand is a dynamic GetMember expression, which can't be assigned via an Expression.Assign. We must instead change the dynamic expression from a GetMember to a SetMember. Those bindings are performed via our Late binders.

We fix the issue by adding a new interface IConvertibleToWritableBinder that can convert the read binder to a write binder.

Fixes #340