intern-soft, find-face/get-face, and facep for determining faces' definedness

[Stephen J. Turnbull]

>>>>> "Miles" == Miles Bader <miles at lsi.nec.co.jp> writes:

    Miles> To be reasonably compatible with Xemacs, it would be
    Miles> necessary to know what exactly a face-object (such as
    Miles> returned by `find-face') is useful for, and I don't.

As an optimization.  You can avoid a table lookup if you have the
object.  We also have truly anonymous temporary faces, which get
garbage collected simply by dropping them on the floor, as they never
get entered into any tables.  Another optimization; I guess you could
use gensyms and key-weak hash tables to get the same effect nowadays.
Offhand I'd guess the other APIs (ie, except for `facep) all accept
either a face name or a face object.

The only tricky thing about find-face is that it needs to be
idempotent:

(Assert (null (find-face (find-face [nope]))))    ; not a symbol
(Assert (null (find-face (find-face nil))))       ; a special non-face symbol
(Assert (null (find-face (find-face (gensym)))))  ; a non-face symbol
(Assert (eq (find-face 'default) (find-face (find-face 'default))))

I think that gives full coverage.  Your definition of find-face should
be fine, except possibly for XEmacs APIs Emacs doesn't have.

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