Mark one-parameter constructors as explicit & use override where possible

This commit marks (hopefully) ever one-parameter constructor as explicit.

It also uses override in (hopefully) all circumstances where a virtual
method is overridden.

There are a very few other minor changes - most of them were necessary
to get everything to compile (like one additional constructor). In one
case I changed an implicit operation to an explicit string conversion -
I think the automatically chosen conversion was much more convoluted.

This took longer than I want to admit but not as long as I feared :)

src/IntSet.h

@@ -12,7 +12,7 @@
 class IntSet {
 public:
 	// n is a hint for the value of the largest integer.
-	IntSet(unsigned int n = 1);
+	explicit IntSet(unsigned int n = 1);
 	~IntSet();
 
 	void Insert(unsigned int i);