Merge `tutorial' and `reference' treatments of the same basic GOOPS

Merge `tutorial' and `reference' treatments of the same basic GOOPS
material

* doc/ref/goops.texi (GOOPS): Move use of (oop goops) here.

  (Class Definition): Merged with `Defining New Classes'

  (Instance Creation): Insert before covering slot options.  Merge in
  material from `Creating Instances'.

  (Slot Options): Merged some better wording and index entries from
  the tutorial version.

  (Slot Description Example): New node, containing the <my-complex>
  material from the tutorial.

  (Methods and Generic Functions, Inheritance): Tutorial sections
  moved into main line of the manual.

* doc/ref/goops-tutorial.texi: Nothing left here now.