Fixed for changes between β1 and β5

- Generator ⇔ GeneratorType
- Sequence ⇔ SequenceType
- Added public access level where needed.
- Dropped + operator in favor of `and` and removed overloading.
- Using new internal sequences for filter and map since Swift’s are no
longer lazy.
- != nil, ??
- Slight naming changes in tests (..<, utf16Count, sorted)
- stride, avoids compiler crash