Fix of #50 - volatile

Volatile checking needs to take all intersections into account; previously these
could be discarded through needsChecking.

Plus several refactorings and additions.

1) Module vals now have Final and Stable flags set
2) All logic around isVolatile is now in TypeOps; some of it was moved from Types.
3) Added stability checking to Select and SelectFromType typings.

Todo: We should find a better name for isVolatile. Maybe define the negation instead under the name
"isRealizable"?.
Added lost comment to isVolatile.
Merge pull request #60 from odersky/fix/#50-volatile

Fix/#50 volatile