Bug fix that caused panic due to double free

A reference from an option wasn't pulled out to variable that lived long
enough to be passed to the FFI.

Also an invalid casting that provoked an undefined behavior (i.e. ub)
from a const pointer to mutable pointer was fixed.

All of them were found and fixed by Chris Tam (@GodTamIt)

Use c_char for char FFI & fix CI lints

Use `c_char` for `char` for FFI to avoid compiling errors in different
architectures.

Fix some linter issues current reported by the CI, likely because of an
update of the linters.

Major bug fix (breaking changes) & other minor fixes

- Fix a bug about free after in an unsafe block that was calling the
uplink-sys API.
- Fix missing documentation of unsafe blocks and add the linter rule to
deny any undocumented unsafe block.
- Fix the indentation in some documentation comments.

uplink-sys version 0.7.1

- Update a bindgen call because the previous was deprecated and a
warning was shown.
- Make the build more resilient in case that a previous build ran and
failed.

It relies on uplink-sys v0.7.0

Bump the uplink-sys version to v0.7.0 which is the last one published
release when this tag was created.

It relies on uplink-c version 1.9.0

Bump the uplink-c version to v1.9.0 which is the last published release
when this tag and uplink-sys version was created.

Bindgen was also bumped to the last published release until the date.

It relies on uplink-sys v0.6.1

Bump the uplink-sys version to v0.6.1 which is the last one published
release when this tag was created

It relies on uplink-c version 1.8.1

Bump the uplink-c version to v1.8.1 which is the last published release
when this tag and uplink-sys version was created.

Fix typo in an enum variant

The enum used for identifying Uplink errors had a variant with a typo.

This release fix the typo in the variant, hence it has breaking changes.

It relies on uplink-sys version 0.6.0

Bump the uplink-sys version to the last published version and adapt the
implementation to support the new changes and features.