Mark Context and Style as final (#995)

These classes are not designed for extension outside of the package, and
have never been so. Make this explicit with `final` modifiers on
`Context` and `Style` along with the package-private subtypes of
`Style`.

Some code relies on `style == p.Style.windows` type checks, which work
as long as `Style` is used like an enum as intended. Making the classes
final makes this pattern explicitly safe because the package only allows
for instances where identity based equality matches the semantics.

Switch the style subclass libraries to `part of` files.