Combining (and obsoleting) isearch-regexp and isearch-word into a new var

[Artur Malabarba]

Currently `isearch-word' is a variable that can be a boolean
(indicating if word searching is on or off) or it can hold a function
(which is called on the search string before passing it to a regexp
search).
Both symbol-searching and char-folding are implemented as function
values of this variable, while word-searching is implemented as a
boolean value even though it could just as well be another function
value.

In addition to that, we also have regexp-searching, which *could* be
just another function value for that variable (the #'identity), but
instead it's implemented as a whole new boolean variable
(`isearch-regexp').

All of these 4 functionalities are mutually exclusive, so it makes
sense for them to be implemented as different values of a single
variable in a consisten manner (all functions).


I'd like to propose that the `isearch-word' variable be aliased to
`isearch-special' (or something like this), and that its use as a
boolean also be obsoleted.
Then also obsolete `isearch-regexp', which is just a special case of
`isearch-special'.

If we don't want an extra var, then we can just use one of the
existing vars for this purpose and obsolete the other.
I'd just like to condense it all in a single variable and obsolete the
boolean usage.