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.

…the node)

The problem is that const_map_to.get(*j->first) compares only the
shared_ptr's, while we need to compare the key itself.

v2: remove const iterator
v3: fix use-after-free due to const reference

Consider the following YAML:

    trait1: &t1
      foo: 1

    trait2: &t2
      foo: 2

    merged:
      <<: *t1
      <<: *t2

yq reports:

    $ yq .merged.foo < /tmp/yaml
    2

while the order that yaml-cpp returns is different, since it will
firstly handle 1, and will not replace it with 2:

    $ util/parse < /tmp/yaml
    trait1:
      ? &1 foo
      : &2 1
    trait2:
      foo: 2
    merged:
      *1 : *2

(Don't mix up "*2" with "2", it is trait1)