[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

src/kernel/kernel_main.cpp

@@ -36,9 +36,8 @@ kernel_main(bootproto::args *args)
         panic::install(args->panic_handler, args->symbol_table);
     }
 
-    logger_init();
-
     cpu_data *cpu = bsp_early_init();
+    mem::initialize(*args);
 
     kassert(args->magic == bootproto::args_magic,
             "Bad kernel args magic number");
@@ -51,7 +50,6 @@ kernel_main(bootproto::args *args)
 
     disable_legacy_pic();
 
-    mem::initialize(*args);
 
     bsp_late_init();