Plan for stabilization

[udoprog]

The following is a non-exhaustive list of what I want to happen before stabilizing musli-core. As for the main musli crate it does not need to be stabilized.

• Would really like associated_type_defaults (https://github.com/rust-lang/rust/issues/29661). This is currently worked around with attribute macros which fills in any missing attributes for a given version but this solution is not very desirable for stabilization.
• Support #[musli(flatten)] (#179) or make sure it can be supported in the future without breaking changes.
• Settle on trait names in musli-core.