Rename precedence aliases to meaningful names

Done with the following script:
https://gist.github.com/lucaswerkmeister/9dd6dfe1ba44a6ebc2f3

Documentation was updated by hand afterwards.

This finally, after 4 months, closes #13.
completeCompilationUnit: Work around ceylon/ceylon-compiler#1728

Strangely, this only broke on the CLI; the IDE seemed to be able to
compile it.