libgit2 can be built with optional, experimental sha256 support. This
allows consumers to begin testing and providing feedback for our sha256
support while we continue to develop it, and allows us to make API
breaking changes while we iterate on a final sha256 implementation.

The results will be `git2-experimental.dll` and installed as
`git2-experimental.h` to avoid confusion with a production libgit2.

Create `git_email_*` which will encapsulate email creation and
application, and `git_email_create_from_commit` in particular, which
creates an email for a single commit.

We avoid abbreviations where possible; rename git_cred to
git_credential.

In addition, we have standardized on a trailing `_t` for enum types,
instead of using "type" in the name.  So `git_credtype_t` has become
`git_credential_t` and its members have become `GIT_CREDENTIAL` instead
of `GIT_CREDTYPE`.

Finally, the source and header files have been renamed to `credential`
instead of `cred`.

Keep previous name and values as deprecated, and include the new header
files from the previous ones.

Introduce `git_apply_tree`, which will apply a `git_diff` to a given
`git_tree`, allowing an in-memory patch application for a repository.

I'm not sure if worktree.h was intentionally left out of git2.h. Looks like an oversight since it is in fact documented.

It is currently unused; it will go into the remote's options.