Log Message:
mdds: Update to 2.1.1

Changelog:
2.1.1
flat_segment_tree
  * added a method that returns a segment range object compatible with
    ranged for loop.
  * added a move constructor and a move assignment operator.
  * added variants of search() and search_tree() that return a result data
    structure that contains the value, the start and end keys of the range.
multi_type_vector
  * added a range adaptor for mdds::mtv::element_block compatible with
    ranged for loop.

2.1.0
general
  * switched to using ax_valgrind_check for running memory tests. This
    introduces additional build targets, such as check-valgrind to run
    the tests under valgrind.
multi_type_vector
  * delayed_delete_vector has been introduced as the new default storage
    type for the element blocks. This storage type is optimized for use
    cases where elements get repeatedly erased from the front of the array,
    by delaying the actual deletion of the elements until much later.
    This reduces the amount of element shifting associated with the element
    deletions, which can be costly.
  * added an additional template parameter to the element block types in
    order to allow the underlying storage type to be specified per element
    type. This can be used to switch between std::vector, std::deque,
    delayed_delete_vector, or any other compatible custom container types.
sorted_string_map
  * made the entry type a template parameter to allow optionally defining
    the keys in the entry values as std::string_view.

2.0.3
general
  * defined clang-format rules, and globally applied them to all active
    source files.

multi_type_vector
  * revised the block position lookup implementation to avoid using
    the internal STL iterators. The new implementation should be able to
    handle invalid position hints more gracefully without potential process
    termination.

2.0.2
multi_type_vector
  * added optional trace function that gets called on every called public
    method.