Even on systems without C99 where long long and stdint are both
missing, we can shim stdint and point it to any compiler-specific
type (i.e long long, _int64, etc.).

Also next is constant suffixes and determining what needs to
include stdint.

Provide a mechanism for system components to register for initialization
and shutdown of the libgit2 runtime.

In commit bbf034ab9 (hash: move `git_hash_prov` into Win32 backend,
2019-02-22), the `git_hash_prov`'s structure name has been removed in
favour of its typedef'ed name. But as we have no CI that compiles with
the WinHTTPS hashing backend right now, it wasn't noticed that the
implementation that uses this struct wasn't changed correctly.

Fix the struct type to make it compile again.

Create a separate `git_hash_sha1_ctx` structure that is specific
to the SHA1 implementation and move all SHA1 functions over to
use that one instead of the generic `git_hash_ctx`. The
`git_hash_ctx` for now simply has a union containing this single
SHA1 implementation, only, without any mechanism to distinguish
between different algortihms.

As a preparatory step to allow multiple hashing APIs to exist at
the same time, split the hashing functions into one layer for generic
hashing and one layer for SHA1-specific hashing. Right now, this is
simply an additional indirection layer that doesn't yet serve any
purpose. In the future, the generic API will be extended to allow for
choosing which hash to use, though, by simply passing an enum to the
hash context initialization function. This is necessary as a first step
to be ready for Git's move to SHA256.

As we will include additional hash algorithms in the future due
to upstream git discussing a move away from SHA1, we should
accomodate for that and prepare for the move. As a first step,
move all SHA1 implementations into a common subdirectory.

Also, create a SHA1-specific header file that lives inside the
hash folder. This header will contain the SHA1-specific header
includes, function declarations and the SHA1 context structure.

The hash source files have circular include dependencies right
now, which shows by our broken generic hash implementation. The
"hash.h" header declares two functions and the `git_hash_ctx`
typedef before actually including the hash backend header and can
only declare the remaining hash functions after the include due
to possibly static function declarations inside of the
implementation includes.

Let's break this cycle and help maintainability by creating a
real implementation file for each of the hash implementations.
Instead of relying on the exact include order, we now especially
avoid the use of `GIT_INLINE` for function declarations.

The structure `git_hash_prov` is only ever used by the Win32 SHA1
backend. As such, it doesn't make much sense to expose it via the
generic "hash.h" header, as it is an implementation detail of the Win32
backend only. Move the typedef of `git_hash_prov` into
"hash/sha1/win32.h" to fix this.