* EntryWriter: Add comments for why ManuallyDrop is OK

This wasn't immediately obvious to me at least.
xref: #376 (comment)

Signed-off-by: Colin Walters <walters@verbum.org>
Co-authored-by: xzfc <5121426+xzfc@users.noreply.github.com>

General code cleanup but the specific rationale here is I was
trying to find the places this was used in the code and
was surprising cross-referencing didn't turn up the unpacking code.

Signed-off-by: Colin Walters <walters@verbum.org>

I don't think this unique-to-us `t!` macro is gaining anything
really over just calling `unwrap()` directly which is *way*
more widely used and idiomatic.

I only converted a few testing functions to start momentum
and hopefully new code can avoid `t!`.

Signed-off-by: Colin Walters <walters@verbum.org>

* Implementation of a PAX header writer

This crate currently supports reading the PAX headers. This adds a way to write
some PAX headers in a Builder.
Also implemented tests to showcase usage

* Pax header handler, switch to not owned data types

Since we can keep the ownership of the data when calling append_pax_extensions, we can use &str and &[u8] according to the lib documentation

The main feature in this release is a new API to write PAX
extensions.