count_while-bug: fix bug in count_while and couht_while1

On a failed prompt call, the position was not being reset to the
position at the start of the count_while or count_while1, rather than
taking into account the consumed input. In practice this would be
difficult to detect as the end of input was reached, except when
checking if the next parser was checking if it was the end of the input!

This commit fixes the bug and adds regression tests.

lib/angstrom.ml

@@ -342,7 +342,7 @@ let rec count_while ~init ~f ~with_buffer =
       let succ' input' pos' more' =
         (count_while ~init:init' ~f ~with_buffer).run input' pos' more' fail succ
       and fail' input' pos' more' =
-        succ input' pos' more' (Input.apply input' pos' init' ~f:with_buffer)
+        succ input' (pos' + init') more' (Input.apply input' pos' init' ~f:with_buffer)
       in
       prompt input pos fail' succ'
   }
@@ -370,7 +370,7 @@ let rec count_while1 ~f ~with_buffer =
       let succ' input' pos' more' =
         (count_while ~init:len ~f ~with_buffer).run input' pos' more' fail succ
       and fail' input' pos' more' =
-        succ input' pos' more' (Input.apply input' pos' len ~f:with_buffer)
+        succ input' (pos' + len) more' (Input.apply input' pos' len ~f:with_buffer)
       in
       prompt input pos fail' succ'
   }

lib_test/test_angstrom.ml

@@ -341,6 +341,14 @@ let incremental =
         (string "thi" *> string "st" *> commit *> string "hat") ["thi"; "st"; "hat"] "hat";
     end ]
 
+let count_while_regression =
+  [ "proper position set after count_while", `Quick, begin fun () ->
+    check_s ~msg:"take_while then eof"
+      (take_while (fun _ -> true) <* end_of_input) ["asdf"; ""] "asdf";
+    check_s ~msg:"take_while1 then eof"
+      (take_while1 (fun _ -> true) <* end_of_input) ["asdf"; ""] "asdf";
+  end ]
+
 let () =
   Alcotest.run "test suite"
     [ "basic constructors"    , basic_constructors
@@ -350,4 +358,6 @@ let () =
     ; "applicative interface" , applicative
     ; "alternative"           , alternative
     ; "combinators"           , combinators
-    ; "incremental input"     , incremental ]
+    ; "incremental input"     , incremental 
+    ; "count_while regression", count_while_regression
+  ]