New features and rewrite of major ECST modules

[vittorioromeo]

• Now uses boost::hana for all metaprogramming code. Compilation times significantly decreased.
• Most of the user syntax for defining signatures and compile-time settings is much easier to read/write and uses method chaining. Explicitly specified namespaces occur much less often in user code.
• Component signatures now allow users to specify a storage type. Signatures can be bound to a variadic amount of component types, easily allowing users to switch between AoS and SoA structures, or allowing developers to create clever data structures that know about component type relationships.
• System execution is much more flexible, allowing users to separately execute chains of systems in multiple steps. Single system executions are more flexible as well, allowing users to perform single actions before/after dealing with system subtasks.
• It is now possible to handle refresh events such as system subscription/unsubscription and entity ID retrieval, without any additional run-time cost. context::step accepts secondary event-handling functions that get overloaded at compile-time and allow users to process the aforementioned events.