Simplify ObjectModel, and try to get rid of the StorageAccessors again

[smarr]

In https://github.com/SOM-st/TruffleSOM/pull/47, TruffleSOM uses a "better" approach to initialize the unsafe accesses for the object model. Instead of needing instances of objects, which have their own virtual methods, we can keep all the logic in one place, and only read the offsets from the StorageAnalyzer.

This might be a benefit for interpreter performance, though, unmeasured, and I don't remember what the impact was for introducing the current design in SOMns.

[smarr]

See https://github.com/SOM-st/TruffleSOM/pull/47/commits/b121f1b3aa587aece66ccf8cac9be28005585fb0 and https://github.com/SOM-st/TruffleSOM/pull/47/commits/fb73434f56e3b21174ab9ee07d222a40650b6d77