Only avoid collision if it's the same file

@chriseth had suggested that it would be better if contracts were
referenced in a file:contract notation, and that we output .bin files
that prepend original path names if necessary to avoid a collision.

This commit is mostly a draft; it still needs to be run through the test
suite.

libsolidity/ast/AST.h

@@ -358,6 +358,8 @@ public:
 	Json::Value const& devDocumentation() const;
 	void setDevDocumentation(Json::Value const& _devDocumentation);
 
+	std::string fullyQualifiedName() const;
+
 	virtual TypePointer type() const override;
 
 	virtual ContractDefinitionAnnotation& annotation() const override;