fix: fix libsass 4.0 call deprecation warning at-import#63

Closes at-import#63

stylesheets/functions/_every.scss

@@ -20,10 +20,10 @@
 
 @function sl-every($list, $function, $args...) {
   @each $item in $list {
-    @if not call($function, $item, $args...) {
+    @if not call(sl-safe-get-function($function), $item, $args...) {
       @return false;
-    } 
+    }
   }
-  
+
   @return true;
 }

stylesheets/functions/_some.scss

@@ -21,13 +21,13 @@
 ///
 /// @return {Bool}
 ///
- 
+
 @function sl-some($list, $function, $args...) {
   @each $item in $list {
-    @if call($function, $item, $args...) {
+    @if call(sl-safe-get-function($function), $item, $args...) {
       @return true;
-    } 
+    }
   }
-  
+
   @return false;
 }

stylesheets/functions/_walk.scss

@@ -18,18 +18,18 @@
 ///
 /// @return {List | Null}
 ///
- 
+
 @function sl-walk($list, $function, $args...) {
   $_: sl-missing-dependencies('sl-to-map', 'sl-to-list');
-  
+
   @if not function-exists($function) {
     @error 'There is no `#{$function}` function for `sl-walk`.';
   }
 
   @each $index, $value in sl-to-map($list) {
     $arguments: join($value, $args);
-    $list: set-nth($list, $index, call($function, $arguments...));
+    $list: set-nth($list, $index, call(sl-safe-get-function($function), $arguments...));
   }
-  
+
   @return sl-to-list($list);
 }