When "send" feature is enabled, make Readers Send too

[commial]

Mimic the behavior of the ArchiveWriter for ArchiveReader when the "send" feature is enabled.

As a result, when the feature "send" is enabled, ArchiveReader is now Send

[commial]

Due to feature unification, when compiling the workspace with cargo --all-features, mla is compiled with the feature Send and, as a result, the mlar's mla dependency is also compiled with these feature (even if not actually requiring it).

In order to keep a cargo build --all-features working in the root workspace, the API used by mlar is updated. This should not be a break of SemVer, as the API is still compatible (if mlar was built inside its own workspace, this problem will not raise).

Finally, as the InnerLayerReaderTrait is not pub in the current crates.io version, the cargo publish fail. This is an expected behavior until a new mla release is made.