Plan for cleaner seperation of libstd facade and implementation

[panicbit]

There are 3 key crates in this plan: libstd, abstract_platform, and platform (e.g. platform_unix).

• libstd links to the abstract_platform and a platform crate. Almost all libstd types wrap around their counterpart in abstract_platform, instancing them via the platform impl.
  This crate needs to make sure to only interact with the platform crate via the abstract_platform types, with the exception of statics and the ext module.
  No assumptions over any of the types provided by platform may be made.
  To allow a smoother transition, this will be implemented completely inside of ::sys at first.

• ‎abstract_platform provides traits deacribing the required types, functions and constants to be provided by a platform crate. Furthermore it provides almost all types of libstd but generic over the platform (using the aforementioned traits). Statics can't be generic, so they can't be formalized via a trait and thus need to follow an informal contract (probably described in the crate doc).

• ‎platform provides a platform implementation by implementing the traits and statics from abstract_platform.
  Platform specific extensions may be provided at ::ext via a platform specific contract.
  This crate needs to take care not to export any items except ::Std (which needs to implement abstract_platform::traits::Std), required statics and an optional ext module.
  ::Std must be either an empty struct or an empty enum.