Add module_mapper_t interfaces to iterate modules without mapping, and to write them out

We have several use cases of manipulating module data for offline drcachesim traces, and since the format includes binary blobs, it is best to provide proper interfaces rather than having hacky scripts make assumptions about the format.

What's missing, specifically, to allow offline rewriting of module data, is:

• A module_mapper_t interface to iterate modules without mapping.
• A module_mapper_t interface to write out module data after it's been changed.
• A way to specify the "print_cb" for custom data (whether the one used with drmemtrace_custom_module_data or a different one).