Minor ungrouped improvements and bug fixes.
Type: make type<> more compiler-friendly
Core: document bug with data type retrieval
Functional: refactor to reduce coupling
Use c-style variadics in at_index::overload_resolution to reduce instantiations.
Type: minor refactoring
Monad: test the minimal complete definitions
List: refactor and use eval_if more consistently
Traits: convert Integrals to their underlying integer explicitly
Integral: make integral<T, n> equivalent to true_ for n >= 1 in a boolean context
Foldable: reduce the mcd, steal find from Iterable and improve tests
Range: convert Integrals to their underlying type explicitly
Functor: documentation style revamp
Core: document and don't require core.hpp for instance and defaults
Core: slightly improve the performance of datatype<>
Maybe: minor refactoring
Comparable: add unit tests for the mcds
Remove obsolete Doxygen aliases
README: document why we don't have forward declaration headers
core: Add conversion between data types