All-open compiler plugin -> Open-By-Default compiler plugin

[yole]

The key reason why we’re never going to ship something like this as an official plugin is that, as discussed in A bit about picking defaults, open by default breaks smart casts. Opening a class via a compiler plugin is even worse because it breaks smart casts silently: the compiler assumes that it can rely on a property of the code when in fact this property can be violated at runtime. In the specific set of cases supported by the kotlin-spring plugin, this is typically not an issue, but this will become an issue if we support it as a general solution.