Change findfirst/findlast/findnext/findprev to return the same index …

…type as keys() (#25577)

* Change findfirst/findlast/findnext/findprev to return the same index type as keys()

This is consistent with find(). We need to check whether the iterator is
empty first, since calling first(keys(A)) may throw an error. Also
add needed last(::EachStringIndex) and prevind(::AbstractArray, ::CartesianIndex) methods.

* Use more generic approach for findfirst and findlast

Also add tests, and remove @inferred calls which did not actually check
whether functions were inferred, but only whether the equality test was.
Change the _pairs() fallback to use Iterators.countfrom() so that an error
is raised by findlast() rather than assuming the last index is typemax(Int),
and use length() for iterators which support it so that findlast() works.

NEWS.md

@@ -376,12 +376,12 @@ This section lists changes that do not have deprecation warnings.
   * `findn(x::AbstractArray)` has been deprecated in favor of `findall(!iszero, x)`, which
     now returns cartesian indices for multidimensional arrays (see below, [#25532]).
 
-  * `find` has been renamed to `findall`, and now returns the same type of indices
-    as `keys`/`pairs` for `AbstractArray`, `AbstractDict`, `AbstractString`, `Tuple`
-    and `NamedTuple` objects ([#24774], [#25545]).
-    In particular, this means that it returns `CartesianIndex` objects for matrices
-    and higher-dimensional arrays instead of linear indices as was previously the case.
-    Use `LinearIndices(a)[findall(f, a)]` to compute linear indices.
+  * `find` has been renamed to `findall`. `findall`, `findfirst`, `findlast`, `findnext`
+    now take and/or return the same type of indices as `keys`/`pairs` for `AbstractArray`,
+    `AbstractDict`, `AbstractString`, `Tuple` and `NamedTuple` objects ([#24774], [#25545]).
+    In particular, this means that they use `CartesianIndex` objects for matrices
+    and higher-dimensional arrays insted of linear indices as was previously the case.
+    Use `LinearIndices(a)[findall(f, a)]` and similar constructs to compute linear indices.
 
  * `AbstractSet` objects are now considered equal by `==` and `isequal` if all of their
     elements are equal ([#25368]). This has required changing the hashing algorithm