Change EmptyLayerCause::{InvalidMetadataAction,RestoredLayerAction} to be tuple enum variant

[edmorley]

Currently EmptyLayerCause::InvalidMetadataAction and EmptyLayerCause::RestoredLayerAction are defined as struct enum variants with a single cause field:

libcnb.rs/libcnb/src/layer/struct_api/mod.rs

Lines 81 to 94 in 3fa2c6c

	pub enum EmptyLayerCause<MAC, RAC> {
	/// The layer wasn't cached in a previous buildpack run and was newly created.
	NewlyCreated,
	/// The layer was cached in a previous buildpack run, but the metadata was invalid and couldn't
	/// be converted into a valid form. Subsequently, the layer was deleted entirely.
	///
	/// See: `invalid_metadata_action` in [`CachedLayerDefinition`].
	InvalidMetadataAction { cause: MAC },
	/// The layer was cached in a previous buildpack run, but the `restored_layer_action` function
	/// rejected the contents and/or metadata.
	///
	/// See: `restored_layer_action` in [`CachedLayerDefinition`].
	RestoredLayerAction { cause: RAC },
	}

This cause field is derived from the return value from the invalid_metadata_action and restored_layer_action functions.

However, I don't think it's ideal to name it cause, since:

1. The embedded result/payload returned from invalid_metadata_action and restored_layer_action can be anything - it doesn't have to be related to the "cause" at all. eg: A layer could be implemented to always return RestoredLayerAction::DeleteLayer no matter what, but include some piece of metadata from the old layer (eg the runtime version from the last successful build) for use outside of restored_layer_action. In which cause calling it "cause" is not accurate.
2. Even in cases where the returned value from restored_layer_action is technically a "cause", the named struct field of cause isn't accurate when there are multiple reasons for invalidation. For example, I have to rename the field to the plural reasons, which is a bit clunky: EmptyLayerCause::RestoredLayerAction { cause: reasons } (see here).

If these enum variants instead used a tuple type type, the naming of the variable is left up to the author:

EmptyLayerCause::RestoredLayerAction(reasons)

If we do this, there are a few other places where we'd want to replace "cause" references too:

libcnb.rs/libcnb/src/layer/struct_api/mod.rs

Lines 70 to 107 in 3fa2c6c

	pub enum LayerState<MAC, RAC> {
	/// The layer contains validated cached contents from a previous buildpack run.
	///
	/// See: `restored_layer_action` in [`CachedLayerDefinition`].
	Restored { cause: RAC },
	/// The layer is empty. Inspect the contained [`EmptyLayerCause`] for the cause.
	Empty { cause: EmptyLayerCause<MAC, RAC> },
	}
	/// The cause of a layer being empty.
	#[derive(Copy, Clone, Debug, Eq, PartialEq)]
	pub enum EmptyLayerCause<MAC, RAC> {
	/// The layer wasn't cached in a previous buildpack run and was newly created.
	NewlyCreated,
	/// The layer was cached in a previous buildpack run, but the metadata was invalid and couldn't
	/// be converted into a valid form. Subsequently, the layer was deleted entirely.
	///
	/// See: `invalid_metadata_action` in [`CachedLayerDefinition`].
	InvalidMetadataAction { cause: MAC },
	/// The layer was cached in a previous buildpack run, but the `restored_layer_action` function
	/// rejected the contents and/or metadata.
	///
	/// See: `restored_layer_action` in [`CachedLayerDefinition`].
	RestoredLayerAction { cause: RAC },
	}
	/// A value-to-value conversion for layer actions.
	///
	/// Similar to [`Into`], but specialized. Allowing it to also be implemented for
	/// values in the standard library such as [`Result`].
	///
	/// Implement this trait if you want to use your own types as actions.
	///
	/// libcnb ships with generic implementations for the majority of the use-cases:
	/// - Using [`RestoredLayerAction`] or [`InvalidMetadataAction`] directly.
	/// - Using [`RestoredLayerAction`] or [`InvalidMetadataAction`] directly, wrapped in a Result.
	/// - Using [`RestoredLayerAction`] or [`InvalidMetadataAction`] with a cause value in a tuple.
	/// - Using [`RestoredLayerAction`] or [`InvalidMetadataAction`] with a cause value in a tuple, wrapped in a Result.