[fuchsia.net] Restore fuchsia.net.MacAddress

This is used in Garnet, and we're not ready to transition to the
zircon.ethernet definition.

Test: local build with Garnet
Change-Id: I052a9e302cc77baeb52f7b99d371eb61dac070f0

[fidl] Add ethernet interfaces

These will be used in garnet to allow FIDL-based ethernet device creation.

Test: used in garnet locally.

Change-Id: Ifd932217e91ac9da4ed948e871d053fa26ccdea4

[fidl] Rename FIDL directories

These names more closely match what we publish in the SDK. Ideally, we'd
use . as the separator, but Zircon's build system cannot handle . in
directory names, so we use - instead.

Test: Builds.
Change-Id: Ia420a364d2accac39faebade8f3f8ed72088556a