Add debug string and ODesc support to HashKey class

This allows tracing of hash key buffer reservations, reads, and writes via a new
debug stream, and supports printing a summary of a HashKey object via
Describe(). The latter comes in handy e.g. in TableVal::Describe() (where
including the hash key is now available but commented out).

src/Hash.h

@@ -28,8 +28,12 @@
 
 namespace zeek
 	{
+
 class String;
+class ODesc;
+
 	}
+
 namespace zeek::detail
 	{
 
@@ -354,6 +358,8 @@ public:
 	const void* KeyAtRead() const { return static_cast<void*>(key + read_size); }
 	const void* KeyEnd() const { return static_cast<void*>(key + size); }
 
+	void Describe(ODesc* d) const;
+
 protected:
 	char* CopyKey(const char* key, size_t size) const;