archive, chrootarchive: split out decompression

In `ApplyLayer` and `Untar`, the stream is magically decompressed. Since
this is not able to be toggled, rather than break this ./pkg/ API, add
an `ApplyUncompressedLayer` and `UntarUncompressed` that does not
magically decompress the layer stream.

Signed-off-by: Vincent Batts <vbatts@redhat.com>