Associate definitions of accessor generic functions with the corresponding class definitions

[robert-strandh]

The new bootstrapping procedure loads definitions of generic functions and definitions of classes into the same environment, even though the class definitions end up in a different environment from the generic functions. This new organization therefore no longer requires the separation of definitions of accessor generic functions and the class definitions that refer to those accessors.

We should therefore systematically eliminate the separate files that now contain definitions of accessor generic functions, and instead put those definitions in the same file as the associated class definitions.