Use the 'default' keyword where applicable. This patch

Use the 'default' keyword where applicable. This patch
 was initially created using clang-tidy:

  clang-tidy -checks="-*,moderize-use-default" -fix

However, the resulting patch only wanted to alter the
definitions in the .cpp file. I think it makes more
sense to remove the definitions completely and just
add "= default" to the declarations in the header file.