Fix the IR type of array updates.
Fix the IR type of conversions to Byte/Short.
Add the type of `this` of Function nodes.

After all, it is part of their signature as much as the types
of their parameters.
Record ancestors of hijacked classes in IR.
Fix the mutable flag of VarRefs and Selects.
Fix some IR types with legit Casts.
Fix encoded name of Null and Nothing in signatures.
Fix the IR type of Object, Null and Nothing.
wip