Add a new class SystemProfiler and a default null singleton instance.

This class implements hookpoints of interest to a system profiler,
currently hooking file load and PHP errors.  This list may grow.

The new class also supplies a HotProfiler.

The instance of a SystemProfiler may be made and bound during the initialization
phase of a 3rd party DSO.

While I'm at it, plumb in instantiation of the pre-existing experimental MemoProfiler.
```
   Warning  (FBCPP2) CppLint Warning
     Warning: Base class SystemProfiler has virtual functions but a public
    non-virtual destructor.

              36  * That binding can be done in a dynamic shared object.
              37  */
              38
    >>>       39 class SystemProfiler {
              40 public:
              41   /*
              42    * Called when a PHP execution error is handled.
```