Support for YAML Merge keys ( <<: [*dict1, *dict2] ) is added. The merge
key is a specific scalar with value << (and tag !!merge) that implies
that during node construction, the map (or sequence of maps) are merged
into the current map. The priority rules are that each key from maps
within the value associated with << are added iff the key is not yet
present in the current map (and first map gets higher priority). Test
cases have been added accordingly.
* Add compilation flags: -Wshadow -Weffc++ -pedantic -pedantic-errors
* Delete implicit copy & move constructors & assignment operators
in classes with pointer data members.
* An exception to the above: Add default copy & move constructors &
assignment operators for the Binary class.
* Convert boolean RegEx operators to binary operators.
* Initialize all members in all classes in ctors.
* Let default ctor delegate to the converting ctor in
Binary and RegEx
* Don't change any tests except regex_test (as a result of the change
to binary operators).
Note: https://bugzilla.redhat.com/show_bug.cgi?id=1544675 makes
-Weffc++ report a false positive in "include/yaml-cpp/node/impl.h".
