Add bif that allows searching for all matching subnets in table.

Example:

global test: set[subnet] = {
	10.0.0.0/8,
	10.1.0.0/16,
	10.2.0.0/16,
	10.2.0.2/31
}

print matching_subnets(10.2.0.2/32, test);
->
[10.2.0.2/31, 10.2.0.0/16, 10.0.0.0/8]

src/Val.h

@@ -814,6 +814,11 @@ public:
 	int Size() const	{ return AsTable()->Length(); }
 	int RecursiveSize() const;
 
+	// Returns the Prefix table used inside the table (if present).
+	// This allows us to do more direct queries to this specialized
+	// type that the general Table API does not allow.
+	const PrefixTable* Subnets() const { return subnets; }
+
 	void Describe(ODesc* d) const override;
 
 	void InitTimer(double delay);