Try to detect nullability violations in advance

[mdekstrand]

I think we can do a fair amount of detection of nullability violations in advance. We might already do some of this.

Ideas:

• [ ] Resolving non-nullable dependencies with null bindings should fail.
• [ ] Binding a non-nullable dependency to a provider whose get() method is annotated with @Nullable should emit a warning.
• [ ] Binding a non-nullable dependency to a provider whose get() method is not annotated with @Nonnull: warn or leave alone?