xen/timers: Document and improve the representation of the timer heap metadata

xen/timers: Document and improve the representation of the timer heap metadata

The {GET,SET}_HEAP_{SIZE,LIMIT}() macros implement some completely
undocumented pointer misuse to store the size and limit information.  In
practice, heap[0] is never a timer pointer, and used to stash the metadata
instead.

Extend the HEAP OPERATIONS comment to include this detail.  Introduce a
structure representing the heap metadata, and a static inline function to
perfom the type punning.

Replace all of the above macros with an equivelent expression involving the
heap_metadata() helper.  Note that I deliberately haven't rearranged the
surrounding code - this allows the correctness of the transformation to be
checked by confirming that the compiled binary is identical.

This also removes two cases of a macro argument with side effects, which only
worked correctly because the arguments were only evaluated once.

Finally, fix up the type of dummy_heap.  The old code functioned correctly,
but only by virtue of confusing a discrete object and a single-entry array.
Change its type to match the intended semantics, and drop the redundant
initialisation in timer_init().

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>