Runtime allocation tracking now also tracks the number
of heap allocations, the number of freed heap
allocations and the number of GC iterations via counters.

Additionally, there is now a way to check if runtime
memory allocation tracking is enabled or not via
`ifdef` statements in Pony code. This allows for some
useful validations for those folks concerned about
heap allocations in the critical path (i.e. if they
rely on the compiler's `HeapToStack` optimization pass
to convert heap allocations to stack allocations and
want to validate it is working correctly).

This commit enhances the runtime stats tracking that was previously
implemented under the `USE_MEMTRACK` and `USED_MEMTRACK_MESSAGES`
defines. The new defines are called `USE_RUNTIMESTATS` and
`USE_RUNTIMESTATS_MESSAGES`.

Runtime stats tracking tracks the following actor info:
* heap memory allocated
* heap memory used
* heap num allocated
* heap realloc counter
* heap alloc counter
* heap free counter
* heap gc counter
* system message processing cpu usage
* app message processing cpu usage
* garbage collection cpu usage
* messages sent counter
* system messages processed counter
* app messages processed counter

Runtime tracking tracks the following scheduler info:
* mutemap memory used
* mutemap memory allocated
* memory used for gc acquire/release actormaps and actors created
* memory allocated for gc acquire/release actormaps and actors created
* created actors counter
* destroyed actors counter
* actor system message processing cpu for all actor runs on the scheduler
* actor app message processing cpu for all actor runs on the scheduler
* actor garbage collection cpu for all actor runs on the scheduler
* scheduler message processing cpu usage
* scheduler misc cpu usage while waiting to do work
* memory used by inflight messages
* memory allocated by inflight messages
* number of inflight messages

This runtime stats tracking info has been exposed to pony programs as
part of the `runtime_info` package and an example `runtime_info` program
has been added to the `examples` directory.