Introduce Directive Hierarchy (#76)

* Introduce Directive Hierarchy

to classify grammatical declarations that modify the behavior of a C++ translator

* Remove `DeclSort::UsingDirective`

in favor of defined `DirSort::Using`

* Add a field for the result of `DirSort::DeclUse`

* Add `specifiers` and `access` to `DeclSort::Barren`

* Clarify `DirExpr::Using` and `DirSort::DeclUse`

* Embed directives in `StmtSort`

* Update `Phases`

* Introduce directive heap

Update `DirSort::Tuple`