Add .mailmap file
This improves the output of git shortlog and others, see
for more details.
The mapping is in part derived from the Darcs author-spelling file
which was removed some time ago via 47a018026aee9faef28ddc9b4550425dd0000ceb.
There's still a couple of unmapped <unknown> authors in the Git history which will
be hopefully mapped in the future as well to provide complete authorship information
throughout all of GHC's recorded development history.
Signed-off-by: Herbert Valerio Riedel <firstname.lastname@example.org>