Add a Makefile so that pEpEngineSequoiaBackend can be build using the
usual pEp style without directly invoking cargo, supporting the usual
targets build, install, uninstall, test, clean, all.
Make the project easy to install and uninstall, with a user-settable
prefix.
The Makefile is extensible with a local.conf file, of which we provide
an example.

Add CI support.

Add a .gitignore file.

Co-authored-by: Heck <heck@pep.foundation>
Co-authored-by: Neal H. Walfield <neal@walfield.org>
Co-authored-by: Luca Saiu <positron@pep.foundation>
Co-authored-by: Devan Carpenter <dvn@pep-project.org>
Co-authored-by: Volker Birk <volker.birk@pep-project.org>

  - Update `Cargo.lock` to get the latest and greatest.  In
    particular, buffered-reader 1.1.4 and sequoia-openpgp 1.13.0.

  - Require chrono 0.4.23 or newer, not exactly 0.4.23.

  - Enable static linking, which Android and iOS builds prefer.

  - See PEMA-103.

  - Fix deprecations.

  - Don't use the `rust-toolchain` file to specify the version of
    `cargo` and `rustc` to use.  Instead use `rust-version` in
    `Cargo.toml`.

  - Note: this changes how we choose the tool chain: `rust-version` is
    a minimal version; `rust-toolchain` is an exact version.

  - Use 1.60 as that is what is currently available in Debian testing.

  - Closes #7, #10.

  - The repository has moved to the pEp.foundation organization.
    Update the URL.

  - Add a function to copy a slice to a C pointer and length.

  - Use it.

  - Add macros to check that pointers are not NULL, to turn a pointer
    and length into a slice, to turn a C string into a `CStr`, and to
    interpret an OpenPGP fingerprint as a `Fingerprint`.

  - Use them.

  - If `PepIdentity::as_mut` is passed a NULL pointer, return an error.

  - Change `rust_str_to_c_str` and `rust_bytes_to_c_str_lossy` to
    return a `Result`.

  - Return `Error::OutOfMemory` if `malloc` fails instead of undefined
    behavior.

  - Update the callers to propagate the error when convenient and to
    panic explicitly when not.

    - Note: Rust panics if a memory allocation fails so we don't go
      too far out of the way to propagate memory allocation errors.

  - Add a struct, MM, which holds the malloc and free implementation
    passed to pgp_init_.

  - Pass it to all functions that allocate using malloc or free memory
    using free.

  - enumber 0.3 has been released.  It is no longer necessary to use
    an unreleased version.

  - Instead of using the cookie in the Session data structure to point
    to the Keystore, point to a struct.

  - This makes it possible to have additional state.

  - stringpair_t and stringpair_list_t will be needed by the
    implementation of pgp_list_keyinfo.

  - Add the GPL v3 to COPYING.

  - Fix the stale package config template.

  - Always generate it when building.

  - What cargo wants, cargo gets.