Standalone Maybe: refactor Variant, Optional, Maybe and more

[PragmaTwice]

oneflow::maybe::Variant:

• high performance .Visit(): unlike std::visit for std::variant, we use a hybrid policy between trivial recursion and binary search to retain full optimization (see https://github.com/PragmaTwice/oneflow/blob/maybe/oneflow/maybe/variant.h#L40 for details)
• unifed experience with Optional: NullOpt for both std::monostate and std::nullopt (see https://github.com/PragmaTwice/oneflow/blob/maybe/oneflow/maybe/utility.h#L29 for details)
• no std::shared_ptr, unlike SharedOrScalar, EitherPtr in oneflow

oneflow::maybe::Optional:

• optimized storage for scalar type
• support reference type, unlike std::optional
• support monadic operations Map, AndThen and OrElse
• hide .Value() (only exposed to JUST via friend class), unlike std::optional (cleaner than oneflow::Optional)
• no std::shared_ptr, unlike oneflow::Optional for non-scalar type

flexiable stacked error types

• StackedErrorTraits<T> for a type T to become a StackedError
• simple::StackedError and simple::NoStackError as demo error types
• and even std::unique_ptr<simple::StackedError> can be an Error because we have StackedErrorTraits<std::unique_ptr<T>>
• define your own stacked error types easily (just need some member functions OR provide StackedErrorTraits<your type>)

oneflow::maybe::Maybe:

• support custom error type
• hide .Value() (only exposed to JUST via friend class)
• support monadic operations Map, AndThen
• no std::shared_ptr, unlike oneflow::Maybe for non-scalar type (use Maybe<std::shared_ptr<T>, E> if you want shared_ptr)

customizable JUST macro:

• support Maybe and Optional
• customizable via JustTraits, then we can write JUST(some_shared_ptr)
• NO GLOG REQUIRED, you can use your favorite logging library, and just need to define some behavior in error types