kernel: Add k_heap synchronized memory allocator

This adds a k_heap data structure, a synchronized wrapper around a
sys_heap memory allocator.  As of this patch, it is an alternative
implementation to k_mem_pool() with somewhat better efficiency and
performance and more conventional (and convenient) behavior.

Note that commit involves some header motion to break dependencies.
The declaration for struct k_spinlock moves to kernel_structs.h, and a
bunch of includes were trimmed.

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

kernel/CMakeLists.txt

@@ -8,6 +8,7 @@ add_library(kernel
   fatal.c
   idle.c
   init.c
+  kheap.c
   mailbox.c
   mem_slab.c
   mempool.c