Make IriExpression internal constructor public

[kal]

There is an internal constructor on the IriExpression class that provides a shortcut for creating an IriExpression from a Uri instance. Making this public would allow developers to do new IriExpression(someUri) instead of new IriExpression(new ConstantTerm(new UriNode(null, someUri)))

[kal]

Note that the URINode constructor is also internal. Should there be some factory method instead of opening up internal constructors? IriExpression.Create(someUri) ?