Shade everything in a subpackage

[propensive]

When building a release version of Fury, it should be possible to write package fury at the start of every source file and everything will be put into a fury-prefixed subpackage which will be visible everywhere inside Fury's source, which is all in the fury package. These packages will then be invisible (except when importing fury) from inside the Scala compiler running inside Fury.

Some care must be taken not to use the _root_ prefix anywhere, though.

[propensive]

This is becoming increasingly important.