Span: use size_t' for index_type, add front' and `back'. (#4935)

* Use `size_t' for index_type.  Add `front' and `back'.

* Remove a batch of `static_cast'.

src/common/hist_util.h

@@ -343,7 +343,7 @@ struct GHistIndexBlock {
 
   // get i-th row
   inline GHistIndexRow operator[](size_t i) const {
-    return {&index[0] + row_ptr[i], detail::ptrdiff_t(row_ptr[i + 1] - row_ptr[i])};
+    return {&index[0] + row_ptr[i], row_ptr[i + 1] - row_ptr[i]};
   }
 };