(apropos-documentation): Don't try to parse .elc files

(apropos-documentation): Don't try to parse .elc files

The old code scanned for #@ in .elc files, assuming they're
docstrings and then looking around them to try and guess to which
definition that docstring belongs, making many assumptions about how
the code happens to be layed out by bytecomp.
Replace that with code which relies on the (FILE . POS) info to
extract the docstring knowing already where they are and what def they
belong to.

* lisp/apropos.el (apropos-documentation-check-elc-file): Delete function.
(apropos--documentation-add-from-elc): New function to replace it.
(apropos--documentation-add): New function, extracted from
`apropos-documentation`.
(apropos-documentation): Use them.  Let-bind `apropos-accumulator` and
`apropos-files-scanned`.
(apropos-documentation-internal): Don't handle the `cons` case any more.
(apropos-item): Don't declare as global var.
(apropos-documentation-check-doc-file): Use `apropos-item` as a local
var rather than a global var.
(apropos-print-doc): Receive `apropos-item` as arg rather than refer to
it as a global variable.
(apropos-print): Adjust calls accordingly.