kernel: Scheduler rewrite

This replaces the existing scheduler (but not priority handling)
implementation with a somewhat simpler one.  Behavior as to thread
selection does not change.  New features:

+ Unifies SMP and uniprocessing selection code (with the sole
  exception of the "cache" trick not being possible in SMP).

+ The old static multi-queue implementation is gone and has been
  replaced with a build-time choice of either a "dumb" list
  implementation (faster and significantly smaller for apps with only
  a few threads) or a balanced tree queue which scales well to
  arbitrary numbers of threads and priority levels.  This is
  controlled via the CONFIG_SCHED_DUMB kconfig variable.

+ The balanced tree implementation is usable symmetrically for the
  wait_q abstraction, fixing a scalability glitch Zephyr had when many
  threads were waiting on a single object.  This can be selected via
  CONFIG_WAITQ_FAST.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

include/sched_priq.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2018 Intel Corporation
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#ifndef _sched_priq__h_
+#define _sched_priq__h_
+
+#include <misc/util.h>
+#include <misc/dlist.h>
+#include <misc/rb.h>
+
+/* Two abstractions are defined here for "thread priority queues".
+ *
+ * One is a "dumb" list implementation appropriate for systems with
+ * small numbers of threads and sensitive to code size.  It is stored
+ * in sorted order, taking an O(N) cost every time a thread is added
+ * to the list.  This corresponds to the way the original _wait_q_t
+ * abstraction worked and is very fast as long as the number of
+ * threads is small.
+ *
+ * The other is a balanced tree "fast" implementation with rather
+ * larger code size (due to the data structure itself, the code here
+ * is just stubs) and higher constant-factor performance overhead, but
+ * much better O(logN) scaling in the presence of large number of
+ * threads.
+ *
+ * Each can be used for either the wait_q or system ready queue,
+ * configurable at build time.
+ */
+
+struct k_thread;
+
+struct k_thread *_priq_dumb_best(sys_dlist_t *pq);
+void _priq_dumb_remove(sys_dlist_t *pq, struct k_thread *thread);
+void _priq_dumb_add(sys_dlist_t *pq, struct k_thread *thread);
+
+struct _priq_rb {
+	struct rbtree tree;
+	int next_order_key;
+};
+
+void _priq_rb_add(struct _priq_rb *pq, struct k_thread *thread);
+void _priq_rb_remove(struct _priq_rb *pq, struct k_thread *thread);
+struct k_thread *_priq_rb_best(struct _priq_rb *pq);
+
+#endif /* _sched_priq__h_ */