refactor(Observable): Update property and method types

[benlesh]

• Removes _isScalar as it was unused
• makes lift protected. This is an internal implementation detail.
• makes source protected, this is an internal implementation detail.
• Refactors operators to use new utility functions that do the lift or provide a reasonable error if the observable someone is trying to use with the operator does not have a lift method. Adds documentation.

BREAKING CHANGE: lift no longer exposed. It was NEVER documented that end users of the library should be creating operators using lift. Lift has a variety of issues and was always an internal implementation detail of rxjs that might have been used by a few power users in the early days when it had the most value. The value of lift, originally, was that subclassed Observables would compose through all operators that implemented lift. The reality is that feature is not widely known, used, or supported, and it was never documented as it was very experimental when it was first added. Until the end of v7, lift will remain on Observable. Standard JavaScript users will notice no difference. However, TypeScript users might see complaints about lift not being a member of observable. To workaround this issue there are two things you can do: 1. Rewrite your operators as outlined in the documentation, such that they return new Observable. or 2. cast your observable as any and access lift that way. Method 1 is recommended if you do not want things to break when we move to version 8.