This adds _clippy.ELFFile, which provides a fast wrapper around libelf.
The API is similar to / a subset of pyelfutils, which unfortunately is
painfully slow (to the tune of minutes instead of seconds.)
The idea is that xrefs can be read out of ELF files by reading out the
"xref_array" section or "FRRouting/XREF" note.
Signed-off-by: David Lamparter <equinox@diac24.net>
