A position now has a start, an end, and a point (the existing `offset`),
just like it does in the Scala compiler. This information is especially
useful for displaying squiggly lines in an IDE.
This commit and the next one are required for https://github.com/sbt/zinc/pull/571
`F0`, `F1` and `Maybe` have become useless since Java 8 introduced
`Supplier`, `Function` and `Optional` in the default Java 8 standard
library.
Therefore, they are not necessary anymore. This change is required to
change some Zinc's and sbt APIs. They are not widely used, so the
changes will be small.
* Make Nothing a singleton
* Make Maybe's constructor private
* Optimise equals to use reference equality first
* Optimise Just.equals by having proper, non-anonymous subclasses
* Having non-anonymous subclasses makes them have nicer classnames
* Give Just a value() method