Add simple profiling class to accumulate Stmt usage stats across runs.

Use the BROFILER_FILE environment variable to point to a file in
which Stmt usage statistics from Bro script-layer can be output.

This should be able to be used to check Bro script coverage that
that e.g. the entire test suite covers.

src/Stmt.h

@@ -52,6 +52,7 @@ public:
 
 	void RegisterAccess() const	{ last_access = network_time; access_count++; }
 	void AccessStats(ODesc* d) const;
+	uint32 GetAccessCount() const { return access_count; }
 
 	virtual void Describe(ODesc* d) const;