add reuse time tool to drmemtrace

Created by: s-kanev

This issue covers creating a "reuse time" tool, which just calculates # references between consecutive accesses to the same address, not # unique addresses in between. Could either be a separate (and cheaper) tool, or just an option to reuse_distance_t.

Most importantly, there should be a programatic way to access the output, so we can use it in downstream tools outside of the DR framework. I can take a stab at the implementation, but I'm not sure what's the best interface change to analysis_tool_t to allow for this. Any ideas?