* while class templates enable e.g. hiding implementation details they also require evaluation via `Eval`
** this clutters up the actual logic and is now hidden behind aliae that perform the evaluation

* replaces `typename *::type` constructs with `Eval` applications
* aims to further unify function evaluation

* `type` is used to represent evaluation of a template
* accessing the `function` doesn't evaluate it as it is the template itself

* analogously to `IsPair`

* moved internals into separate header i.e. the `detail` namespace relating to `Apply`
* implemented automatic alias selection by implementing aliae of the basic variadic `type` template alias in different base classes
** variadic partial application is implemented in `detail::apply_variadic`
*** `detail::apply_single` and `detail::apply_pair` define aliae to `detail::apply_variadic`'s `type` template alias
*** both restricted aliae derive from `detail::apply_variadic`
** `Apply` derives from any of the aliae defining base classes depending on the count of placeholders as determined by `detail::count_placeholders`
*** `Apply` is guaranteed to always be derived from `detail::apply_variadic` one way or the other
* changed functions, test cases and examples depending on `Apply` accordingly
** `Length` had to be reimplemented without `Apply` as it doesn't allow usage of aliae expecting a different count of arguments anymore
*** this is a advantage in the sense that core functionality of _TypeAsValue_ now doesn't depend on this complex partial application implementation anymore
*** such functionality may be reimplemented separately from `Apply`
* removed unnecessary `tav` namespace prefixes

* this is analogous to _Scheme_ where a pair (dot-expression) is returned from a call to `cons`
* `Head` and `Tail` are kept as direct references to the `CAR` and `CDR` values of a pair to match e.g. the math operators