[kernel] Move log buffer to its own memory section

In prep for the coming change to keep log entries as a true ring buffer,
move the log buffer from bss into its own memory section.

Related changes in this commit:
- New vm_area_ring, which maps a set of pages twice to allow easy linear
  reading of data from a ring buffer when it wraps around the end.
- logger_init() went away, and the logger ctor is called from
  mem::initialize()
- Instead of an event object, the logger just has a bare wait_queue
- util::counted::from template type changed slightly to allow easy
  conversion from an intptr_t as well as a pointer
- Previously added debugcon_logger code removed - this will be added in
  a separate file in a followup commit

definitions/memory_layout.yaml

@@ -24,3 +24,6 @@
 
 - name: buffers
   size: 64G
+
+- name: logs
+  size: 2G