Migration: support instance variable rename

[MarcusDenker]

When istance variables are renamed during development, but data is already stored in the database, we will currently lose the data of the renamed ivar, as it is seen as a new ivar + remove ivar.

We need to somehow model the "ivar rename" refactoring

This is not that easy:

• Git does not model any data that knows that an ivar was renamed
• There is some support in Epicea
  • but epicea data is just stored on the local disk of the developer.
  • currently this seems to be disabled (no refactoring is shown in the history when using the RBRenameInstanceVariableRefactoring)

[MarcusDenker]

One possibility would be to use a special Slot class that encodes the meta data. ("this slow is named "x" but is used to be "z"). Similar to #102 , one would need to keep this odd slot until one is sure that the database is updated.

[MarcusDenker]

We can add a RenamedSlot

-> it should be virtual (subclass of Slot, nit IndexedSlot, thus it takes no offset) -> when writing, it should reflectivey write to the new variable -> it should not be serialized in Soil

The idea is that the code has no reads/writes to that value anymore, thus we do not care about reading.

-> database has a value for the old name => this implies no value for the new

-> we read the old name from the DB and reflectively write it -> value ends up in the new variables

When we now write to the database, it will write the value for the new name and not store the RenamedSlot.

=> Database is from now in a good state.