Better output for missing allocatiosn

[itamarst]

Current output is too verbose (it's per allocation!) and uninformative (not useful for debugging, and not actionable for user).

Insetad, we should:

1. Always output a single warning, "X% (Y bytes) of allocations disappeared somehow. If this is a sufficiently small number you can just ignore this warning. You can rerun after doing 'export FIL_DEBUG=1' to get more information, and then file a bug at ..."
2. When FIL_DEBUG is set, print tracebacks for each allocation that goes missing.

Spin off from #149.