Very very old versions of D (well into 0.x) had imports public by default,
like C header files. This modernizes the codebase and removes the
redundant `private` access specifier.
This has been done with:
sed "s/private import/import/g" -i **/*.d
This includes deprecating std.c.*, which apparently was marked as
scheduled for deprecation in 2.068 but never actually deprecated (though
it looks like it was previously removed from the documentation build,
since it doesn't show up on dlang.org).