DrCallstack: provide callstack walking and packing as a DR Extension

This is a longstanding feature request to take Dr. Memory's existing callstack walking code, which has many heuristics for stack scanning along with a compact storage scheme, and turn it into a general library that any DR client can use.

The original entry in the prior tracker:

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on March 09, 2012 09:40:40_

xref issue #822 another extension from the original discussion was DrCallstack: walking and packing callstacks, including md5 and crc32 checksums. again, this would stay within the drmemory codebase: simply build callstack.c as an Extension and perhaps tweak the interface a little if necessary but it shouldn't need much as its options, etc. are all already modular.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=823_