Further validation refactoring

- rename `validation` package back to original name of
  `validator`
  - attempting to follow the guideline of naming a package
    based on what it provides, not what it does
- drop "Must" prefix from validation methods in an effort to make the
  method names sound more natural
- add Validator.NoNilValuesInCollection method
- add validation handling for ColumnItems type alias ([]*Element)
- use Validator methods for validation of Column type

Further refactoring could be beneficial, as would further conversion
of Validate method implementations to use of the validator package.

refs GH-157