Support Java nullability annotations

[lucaswerkmeister]

Several frameworks have attempted to improve on Java’s nullability support by introducing annotations for something being @Nullable or @NotNull/@NonNull/@Nonnull. We should recognize these annotations, and warn or error if:

• a Java @Nullable value (method return type, or field) is assigned to a Ceylon non-optional value or a Java @NotNull, or
• a Ceylon optional value or a Java @Nullable value is assigned to a Java @NotNull value (method parameter, setter parameter, or field).

Behavior on unannotated elements should not change. Some frameworks define only one annotation because the other is the default, but we can’t detect that.

Test cases / examples: https://gist.github.com/lucaswerkmeister/07556232a9bd060dd593

Annotations to support (suggestion, potentially incomplete; list from Kotlin):

• org.jetbrains.annotation: @Nullable, @NotNull
• android.support.annotation: @Nullable, @NonNull
• javax.annotation: @Nullable, @Nonnull (JSR 305, not in main Java yet apparently)
• edu.umd.cs.findbugs.annotations: @Nullable, @NonNull
• javax.validation.constraints: @Null (“must be null” – wat?), @NotNull
• lombok: @NonNull
• org.eclipse.jdt.annotation: @Nullable, @NonNull – warning, without JS these links redirect you to the main help page for some stupid reason
• org.checkerframework.checker.nullness.qual: @Nullable, @NonNull

Alternatively, we could assume that any annotation named @Nullable, @NotNull, @NonNull, @Nonnull has this meaning. I think that’s probably a reasonable assumption.

[jvasileff]

Minor implementation note: assigning null returned from a function marked @NotNull to a Ceylon optional should not result in a runtime error.

And we probably shouldn't have compile time errors for:

if (exists x = getRequired()) { ... }

[lucaswerkmeister]

Moved to ceylon/ceylon-model#8.