src/: Guess the metaclass early, unless we're explicitly bootstrapping.

src/: Guess the metaclass early, unless we're explicitly bootstrapping.

Otherwise we hit unbound-slot errors if we try to resolve class
initializers.

Introduce an internal property `%bootstrapping' to tell the class
initialization machinery to leave the metaclass slot alone, and use this
in `bootstrap-classes'.  Otherwise, have the `sod-class'
`shared-initialize' method go off and call `guess-metaclass'
immediately.

Move `guess-metaclass' back into `class-make-impl.lisp' and fix up the
documentation stubs.

This partially reverts 981b6fb624186a54320cea34e53e16276aee2bdb.