Add CI using GitHub Actions and GitHub Packages:

* This moves our Linux build containers into GitHub Packages; we will
  identify the most recent commit that updated the docker descriptions,
  and then look for a docker image in libgit2's GitHub Packages registry
  for a container with the tag corresponding to that description.  If
  there is not one, we will build the container and then push it to
  GitHub Packages.

* We no longer need to manage authentication with our own credentials or
  PAT tokens.  GitHub Actions provides a GITHUB_TOKEN that can publish
  artifacts, packages and commits to our repository within a workflow
  run.

* We will use a matrix to build our various CI steps.  This allows us
  to keep configuration in a single place without multiple YAML files.

The xenial image depends on ubuntu:xenial; the bionic one on
ubuntu:bionic.  No need for this to be a variable, that's just
additional (unnecessary) state to manage in the CI setup(s).

Azure Pipelines has a version of zlib hanging out on the filesystem;
avoid trying to use it as it's either 64 _or_ 32 bit, so exactly one of
our builds will fail.

Missing declarations

Over time, we have accumulated quite a lot of functions with missing
prototypes, missing `static` keywords or which were completely unused.
It's easy to miss these mistakes, but luckily GCC and Clang both have
the `-Wmissing-declarations` warning. Enabling this will cause them to
emit warnings for every not-static function that doesn't have a previous
declaration. This is a very sane thing to enable, and with the preceding
commits all these new warnings have been fixed.

So let's always enable this warning so we won't introduce new instances
of them.

The function `git_reference__is_note` is not declared anywhere. Let's
add the declaration to avoid having non-static functions without
declaration.

When compiling libgit2 with -DDEPRECATE_HARD, we add a preprocessor
definition `GIT_DEPRECATE_HARD` which causes the "git2/deprecated.h"
header to be empty. As a result, no function declarations are made
available to callers, but the implementations are still available to
link against. This has the problem that function declarations also
aren't visible to the implementations, meaning that the symbol's
visibility will not be set up correctly. As a result, the resulting
library may not expose those deprecated symbols at all on some platforms
and thus cause linking errors.

Fix the issue by conditionally compiling deprecated functions, only.
While it becomes impossible to link against such a library in case one
uses deprecated functions, distributors of libgit2 aren't expected to
pass -DDEPRECATE_HARD anyway. Instead, users of libgit2 should manually
define GIT_DEPRECATE_HARD to hide deprecated functions. Using "real"
hard deprecation still makes sense in the context of CI to test we don't
use deprecated symbols ourselves and in case a dependant uses libgit2 in
a vendored way and knows it won't ever use any of the deprecated symbols
anyway.

We're missing some header includes leading to missing function
prototypes. While we currently don't warn about these, we should have
their respective headers included in order to detect the case where a
function signature change results in an incompatibility.

We've accumulated quite some functions which are never used outside of
their respective code unit, but which are lacking the `static` keyword.
Add it to reduce their linkage scope and allow the compiler to optimize
better.

We have some functions which aren't used anywhere. Let's remove them to
get rid of unneeded baggage.