Support debuginfod for just-in-time metadata creation

[stephenrkell]

The concept of debuginfod is a great fit for liballocs: rather than requiring users to pre-generate the -meta.so file for each binary that gets loaded, the debug info could be sourced on demand and the -meta.so file generated dynamically. This would most obviously mean doing it at load time, although a better approach would be asynchronous and/or would delay until the first query or notification that requires the information.

One problem is that our libdwarf-based tools are currently far too slow: it can take minutes to build the -meta.so for a medium-sized library. The plan to update libdwarfpp to use libdw, which is much faster... it may or may not be fast enough to make this acceptable but something could probably be made to work.