Make MLA Send

[commial]

Send is a trait usually auto-derive.

It seems that, due to Box<dyn> usage of ArchiveWriter and ArchiveReader, these structures are unable to auto-derive the Send trait.

Here a some of the possibles solutions:

1. Realize that the Box<dyn Layer> is too generic, and that actually only a few Layers exist. The Box can then be replaced with an enum of possible layer, hopefully giving enough hint to the compiler to auto-derive Send

• Pro: auto-derive of the trait ; might simplify the code
• Con: likely avoid other crates to add custom Layer (actually not feasible for now due to crate visibility) ; code refactoring needed

2. Explicit Send implementation

• Pro: small changes to the current code base
• Con: unsafe code ; some care must be taken to ensure no unwanted behavior is added. For instance, what if the inner Write is not Send?

3. Like 2., but explicit Send for each layer separately to be more conservative

• Pro: might limits unwanted effects
• Con: as 2. + might not be enough ; more change are required