Move unix implementation to a sub-package, promote inmem interfaces to top-level

[Signed-off-by: Solomon Hykes <solomon@docker.com>]
[Signed-off-by: Paul Nasrat <pnasrat@gmail.com>]
Start to move unix implementation to new beam interface.

Nested streams not yet supported, only supporting mode 0.

[Signed-off-by: Solomon Hykes <solomon@docker.com>]
[Signed-off-by: Paul Nasrat <pnasrat@gmail.com>]
Add support for nested streams in beam/unix

[Signed-off-by: Solomon Hykes <solomon@docker.com>]
[Signed-off-by: Paul Nasrat <pnasrat@gmail.com>]
Test that beam/utils.StackSender works with beam/unix

[Signed-off-by: Solomon Hykes <solomon@docker.com>]
[Signed-off-by: Paul Nasrat <pnasrat@gmail.com>]
Merge pull request #5 from docker/unix

Expose the unix transport under the new interface