Since the unsafe iterator types are now used throughout the overlay, not just by `CxxSequence` and `CxxRandomAccessCollection`, let's move them to a separate file.
This makes `CxxConvertibleToCollection` the base protocol in the hierarchy. Both `CxxSequence` and `CxxRandomAccessCollection` now inherit from `CxxConvertibleToCollection`.
1. Instead of storing the C++ collection within `class CxxIterator`, store a boxed C++ collection and make `CxxIterator` a struct. This enables a number of Swift optimizations for iterators, while preserving the guarantee that the C++ collection is not copied or moved in memory (which would invalidate iterators).
2. Make `sequence` parameter shared. This avoids a second copy of the C++ collection when initializing `CxxIterator`.
Credits to Alex Lorenz for this idea!
This is needed for automatic synthesis of conformances to `CxxSequence` protocol.
It also makes typechecker errors easier to understand when they happen.
This change adds basic helper protocols and structs that are going to be used for making C++ sequences and collection safe and Swifty by adding conformances to `Swift.Sequence`, `Swift.Collection`, etc.
This is not meant to be a final design.
