Revert "const variant is not needed"

This reverts commit 431b0f1.

Author: wusatosi

include/beman/inplace_vector/inplace_vector.hpp

@@ -69,6 +69,7 @@ struct inplace_vector_type_based_storage {
   inplace_vector_array_type<T, Capacity> elems{};
 
   constexpr T *begin() { return elems.data(); }
+  constexpr const T *begin() const { return elems.data(); }
 };
 
 // byte array based storage is used for non-constexpr environment, where default
@@ -80,6 +81,9 @@ struct inplace_vector_bytes_based_storage {
   alignas(T) inplace_vector_array_type<std::byte, Capacity * sizeof(T)> elems;
 
   T *begin() { return std::launder(reinterpret_cast<const T *>(elems)); }
+  const T *begin() const {
+    return std::launder(reinterpret_cast<const T *>(elems));
+  }
 };
 
 //  Base class for inplace_vector