Replace unpickle macro in trait Pickle with extension method

- Removes `PickleFormatType` type member from trait `Pickle`.
- Makes unpickling API symmetric with pickling API.
- Removes unused method `Macro#pickleFormatType` and method `Macro#innerType`.
- Removes unused code in object `Tools`.
Merge pull request #196 from scala/replace-unpickle-macro

Replace unpickle macro in trait Pickle with extension method