The IsLocalhost utility was not used, which only leaves the IsIPv4Localhost
utility.

Go's "net" package provides a `IsLoopBack()` check, but it checks for both
IPv4 and IPv6 loopback interfaces. We likely should also do IPv6 here, but
that's better left for a separate change, so instead, I replicated the IPv4
bits from Go's net.IP.IsLoopback().

Signed-off-by: Sebastiaan van Stijn <[email protected]>

It's the only location it's used, so we might as well move it there.

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Previously, ioutils imported the crypty/sha256 package, because it was
used by the HashData() utility. As a side-effect of that import, the
sha256 algorithm was registered through its `init()` function.

Now that the HashData() utility is removed, the import is no longer needed
in this package, but some parts of our code depended on the side-effect, and
without this, it fail to recognise the algorithms, unless something else
happens to import crypto/sha256 / crypto/sha512, which made our
tests fail:

```
=== Failed
=== FAIL: reference TestLoad (0.00s)
    store_test.go:53: failed to parse reference: unsupported digest algorithm

=== FAIL: reference TestSave (0.00s)
    store_test.go:82: failed to parse reference: unsupported digest algorithm

=== FAIL: reference TestAddDeleteGet (0.00s)
    store_test.go:174: could not parse reference: unsupported digest algorithm

=== FAIL: reference TestInvalidTags (0.00s)
    store_test.go:355: assertion failed: error is not nil: unsupported digest algorithm
```

While it would be better to do the import in the actual locations where it's
expected, there may be code-paths we overlook, so instead adding the import
here temporarily. Until the PR in go-digest has been merged and released.

Signed-off-by: Sebastiaan van Stijn <[email protected]>

This information was already documented in the package's GoDoc, so
no need to repeat it.

Signed-off-by: Sebastiaan van Stijn <[email protected]>