Add statistics collection

Add statistics collection

Statistics meassure this metrics.

For each collection:

* Time spent in the malloc that triggered the collection.
* Time spent with the world stopped.
* Time spent doing the collection.
* Memory info before and after the collection: used, free, overhead and
  wasted memory. Used is the memory used by the mutator, free is the
  memory the mutator can request, overhead is the memory used by the
  collector itself and wasted is memory that is not used by either the
  mutator or collector and that can't be requested by the mutator either.

For each malloc() call:

* Time spent.
* Amount of memory requested.
* Attributes of the requested memory.
* A flag to tell if this call triggered a collection.

Statistics collection is controlled via the D_GC_OPTS environment
variable. To collect malloc statistics, use the option
malloc_stats_file, the value is the path to the file where to store the
malloc statistics (the contents will be replaced). To collect garbage
collection statistics, use the option collect_stats_file, the value is
the path to the file where to store the malloc statistics (the contents
will be replaced). The generated files are in CSV format and have
headers that make them self explanatory.