source-gen: Create file if absent

instead of crashing on the assertion
Port tests to new testing framework
Update import of ANTLR runtime
Update Building section of README.md
Merge branch 'master' into formattingFile-metaParse
Remove obsolete ANTLR dependency
Fully implement formattingOptions using the metamodel.

The necessary method unsafeSet was brought to my attention by @gavinking here:
https://groups.google.com/d/msg/ceylon-users/Yte8e0J3RBo/sDPEUtP0YdAJ
thanks!

This also fixes another appearance of #5.