Replace tarball.LayerFromOpener with stream.NewLayer

[jonjohnsonjr]

The tarball package has methods for reading/writing images from/to the docker save tarball format. This aligns with how the other packages are organized, for example:

• remote is for reading/writing from/to registries.
• daemon is for reading/writing from/to container daemons.
• layout is for reading/writing from/to OCI image layouts.

However, the tarball package also has methods for constructing v1.Layers from a file or from an Opener. It kind of makes sense to be in the tarball package because the layers happen to be compressed tarballs, but really that's an implementation detail. These functions aren't for tarball-specific layers, they're just for layers in general.

We should try to replace these with stream.Layer wherever we can. The tarball.Layer currently reads the whole tarball twice because it eagerly computes the digest/size/diffid. For uncompressed layers, stream.Layer is just better.

For compressed layers, we usually have some metadata that makes eagerly computing those things unnecessary (e.g. remoteLayer). For reading compressed layer from a tarball.Image, we should know the diffids from the config file.

tl;dr LayerFromOpener is bad, let's make stream.Layer work. I elaborated on this a bit here.

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