Fixes deprecated implicit copy warning

[cmey]

Hi and thanks for this wonderful library! I'm proposing a fix for some compilation warnings.

Compiling with warning flag -Wdeprecated-copy (part of -Wextra) produced this type of warning on clang and gcc:

/home/cmey/Code/cpp/ozz-animation/include/ozz/base/span.h:65:8: warning: definition of implicit copy constructor for 'span<const ozz::animation::Float3Key>' is deprecated because it has a user-provided copy assignment operator [-Wdeprecated-copy-with-user-provided-copy]
  void operator=(const span& _other) {
       ^
/home/cmey/Code/cpp/ozz-animation/include/ozz/animation/runtime/animation.h:92:48: note: in implicit copy constructor for 'ozz::span<const ozz::animation::Float3Key>' first required here
  span<const Float3Key> translations() const { return translations_; }

Easy steps to repro on develop:

cmake -D CMAKE_CXX_FLAGS="-Wdeprecated-copy" ../
cmake --build .

I saw 2 occurrences of the warning so there are 2 changes:

• The first in span.h, where I've added the explicit default copy constructor (a copy operator was already implemented). It turns out the copy constructor is used, because explicitly disabling it with = delete produces errors.
• The second in intrusive_list_tests.cc, where I've simply updated the disabling of the copy assignment operator to the more modern way. This way there no longer is a user-provided copy assignment operator, which appeases the warning.

This clears the warnings reported above.

I've followed the Contributing guidelines:

• [x] Ran clang-format on the touched files
• [x] ctest passes locally
• [x] Appended name to AUTHORS.md

Hope this helps!

[guillaumeblanc]

Sorry for the late answer. Many thanks for the pull request and for taking care of following the guidelines !

Cheers, Guillaume

[cmey]

@guillaumeblanc Do you mind making a new release with this fix? maybe a minor version bump to 0.14.1 ? :pray: