DrMalloc: turn malloc interception into a DR Extension

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on March 09, 2012 09:43:47_

xref issue #822 and issue #823 another extension from the original discussion was DrMalloc: malloc/free interception. this would be alloc.c and perhaps also heap.c. would depend on DrSyscall but otherwise is already fairly well-contained.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=824_