Organize and clarify accessors

[sirherrbatka]

As we know, accessor description in CLHS is often missleading, using accessor term exchangable with function (see slot-value function in CLHS). I consider this to be suboptimal approach.

I propose that every accessor should be always named as accessor and also include section describing how it may be implemented. For instance: slot-value *accessor: **reader: function **writer: may expand into function call