Add Tool to Diagnose Whether Corrupt Blobs are Plausibly Bit-Flips

I spend an increasing amount of my time looking at "Corrupted block detected"
failures in decompression. Not infrequently, I suspect that it is the result
of hardware failure, and that the blob has become bit-flipped or otherwise
corrupted somewhere along the line.

For that reason I was motivated to write a little tool to inspect blobs that
fail to decompress, to try modifying them, and then check whether they
decompress successfully. This seems like potentially a generally useful tool,
so I figured it might be worth putting in `contrib/`.

contrib/diagnose_corruption/.gitignore

@@ -0,0 +1 @@
+check_flipped_bits