Add Default derive

[fredszaq]

Resolves #316

This PR depends on #533 (I will mark this PR as non-draft once #533 is merged; for now, you will see changes from both PRs in the diff).

Synopsis

This adds an implementation of a Default derive, with a few improvements over the one bundled with Rust:

• no constraints on generic types by default (useful, for example, if your types only contain Option<T>)
• the ability to set a custom bound on structs/enums via #[default(bound(...))]
• the ability to set default values on fields via #[default(expr(...))]
• the ability to set a non-unit default variant for enums (this needs to be annotated with #[default], as with the standard derive)

Solution

This uses the moved Bounds attribute from #533; otherwise, the implementation is quite straightforward.

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)