Add a feature "send" to provide an Send ArchiveWriter

[commial]

Fix #120

This PR:

• replace the inner type and trait of LayerArchiveWriter with aliases
• introduce a feature send for MLA, disabled by default, which make ArchiveWriter<W> Send
• add a test checking that ArchiveWriter is actually Send when the feature is on

When the feature is on, the W type must be Send.

Forcing Send on the InnerWriterType could have been a simpler way. But it also force the user-provided type to be Sent.
Indeed, the last layer (RawLayer) can't be Send if its inner type (W, the user-provided one) is not Send.

The feature mitigates this behavior: it does not require the W type to be Send, unless the user actually need ArchiveWriter to be Send.

This behavior could have been obtain with an unsafe statement, but I'm trying to avoid them in MLA.