seeker: add a runtime debug stream

This allows detailed but selective debugging when using the library,
particularly when something goes wrong.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/seeker.cc

@@ -0,0 +1,7 @@
+#include "crucible/seeker.h"
+
+namespace crucible {
+
+	thread_local shared_ptr<ostream> tl_seeker_debug_str;
+
+};