Fix backquote on dotted pairs

[SuperDisk]

This commit adds a couple extra cases to backquote's simplification step, which accounts for if the expression is an improper list or dotted pair.

I'm not super happy with this implementation, since I'm not 100% sure these cases account for everything, but it seems to work correctly.

[davazp]

It seems the build failed for it. It might be some bootstrapping issue or just the fix not being correct.

It should be possible to test it in other implementation to see if it is related to bootstrap.