Add mixins and nested rule checking to DeclarationOrder

Change-Id: I3f13160add6513c0f7cd785032d54a4689cb680a
Reviewed-on: http://gerrit.causes.com/43809
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Fix DeclarationOrder linter nits

There were a few things funny about this implementation (mainly the
unnecessary explicit checking of a node within the `check_for_children`
method, which should have simply returned the name of the node type
since it is used in a `map`.

Change-Id: I86342e05890a57c3c96abbc08617a10b082fb445
Reviewed-on: http://gerrit.causes.com/43810
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Simplify some DeclarationOrder specs

These specs seemed more complicated than necessary given what they were
testing. Simplify them.

Change-Id: Iaa05ba339be9ee508bf0d39f121d75a77c017f9a
Reviewed-on: http://gerrit.causes.com/43811
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Update changelog with DeclarationOrder changes

Change-Id: Ie1d3496354fa71c3e656c2b3e10c216fa0e86eed
Reviewed-on: http://gerrit.causes.com/43812
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>