This commit adds adds constants for the networkd config directory, file
prefix.  Network and netdev config each get an associated constant for
the file extension, and each gain a method `write_config_file()` that
writes an appropriately named file to the proper path.

This commit adds a module that contains the structs representing the
network devices we currently configure: interfaces, bonds, and vlans.
These structures are meant to contain the latest versions of all the
underlying config.

This commit adds attributes to the networkd device structs to derive
Deserialize to enable unit testing.  Since we never directly deserialize
into these device structs during normal use, gating the Deserialize
implementation behind the test attribute keeps the binary size a bit
smaller for "normal" builds.

This commit adds a buidler for the systemd-networkd NetDevConfig struct.
The builder contains the logic to translate the structures deserialized
from `net.toml` into NetDev structures representing systemd-networkd
.netdev files.  Since NetDevConfig will be created for bonds and vlans
(and potentially more later), using a builder centralizes the logic to
avoid duplication.  Use a builder also means methods can be limited
based on the type of device being built, providing safety at development
time.

This commit adds a builder for the systemd-networkd NetworkConfig
struct.  The builder contains the logic to translate the structures
deserialized from `net.toml` into NetworkConfig structures representing
systemd-networkd .network files.  Since NetworkConfig will be created
for interfaces, bonds, and vlans (and potentially more later), using a
builder centralizes the logic to avoid duplication.  Using a builder
also means methods can limited based on the type of device being built,
providing safety at development time.