Support viewing help for unbound symbols

[kisaragi-hiu]

An unbound symbol can still have symbol properties. Without this, it is painful to eg. look for where a button type or a face is defined.

The way I merged the display of unbound symbols into the rest of helpful-update is... quite inelegant, but avoiding that would require rearchitecting helpful-update to delegate to different "views" instead, so I've opted to not do that for now.

• helpful--kind-name

  Return "unbound symbol" for symbols that are neither bound as variables or functions.

  Also added this case to its unit test.

• helpful--summary

  Add support for unbound symbols. They are rendered like this: "epg-error is an unbound symbol."

• helpful--bound-p

  Return non-nil for a lambda instead of erroring out.

• helpful-unbound-symbol

  New command. Functionally the same as helpful-variable when called from Lisp, but offers only unbound symbols with properties for completion.

• helpful-symbol

  Call helpful-unbound-symbol for unbound symbols.

• helpful--update

  Add support for unbound symbols.

• helpful--format-references-to-unbound

  New function. This returns the text for the references section. Because unbound symbols do not have symbol-file, this unfortunately has to resort to searching every loaded file with elisp-refs.

• CHANGELOG.md

  Add entry for this change.