Adding #[must_use] on an item

[obi1kenobi]

When #[must_use] is added, code in downstream crates that did not use a value of that type will get a compiler lint.

New lints for existing code requires a minor version: https://github.com/rust-lang/rfcs/blob/master/text/1105-api-evolution.md#minor-change-introducing-new-lint-warningserrors

We want to support both "bare" #[must_use] and the form with a reason: #[must_use = "foo bar"]

[SmolSir]

According to Rust Reference about #[must_use] attribute, the following lints have to be added:

• enum_must_use_added
• struct_must_use_added
• function_must_use_added
• trait_must_use_added
• union_must_use_added
• inherent_method_must_use_added
• trait_method_must_use_added

The following schema additions should be implemented to enable all of the lints (or are closely related to them):

• new schema for Union
• missing implementation for implemented_trait edge for Impl

The union_must_use_added lint requires schema addition before it can be implemented.
The trait_method_must_use_added and inherent_method_moved_must_use_added lints may require schema addition of implemented_trait edge for Impl before they can be implemented.