Fixed handling of nil-literals

Hey there!

This fix seems to bring llgo to a state where it is able to compile the runtime.
Please review thoroughly.

It seems to me that appropriate representation of nil-values is currently missing in the exact-package.
I am using the Basic Kind 'Unknown' to handle that for now.
Another weird thing is that nil-literals are translated to Ident ast-nodes (instead of some kind of constant node).

Cheers,
Anton