ForwardIterable concept documentation page added.
Several new type traits, cleaner type tests.
Fixed various missing includes and forward declarations.
Added inheritable foldl implementation, made most foldable instances users.
ForwardIterable and BackInsertable predicates in prelude.
Several improvements to maybe.

Includes: documentation improvements, ForwardIterable implementation,
made foldl inherited instead of explicitly implemented, ...
Modularised make system for unit tests.

Build times were getting way too long.