Allow usage of `@CheckReturnValue` on types

[odrotbohm]

@CheckReturnValue currently only targets ElementType.METHOD. IDEs, however, also consider a type-level @CheckReturnValue to express that all methods' return values should be checked. I would appreciate it if our annotation could be used that way, too.

It would also make sense to allow the usage of the annotation on constructors and packages, the latter implying all methods in the package having to be checked.