Currently, the Humility CLI accepts IP addresses as a `String`, and then
attempts to parse them as an `Ipv6Addr` and scope. This is a bit
unfortunate, as when argument parsing is performed by `clap`, the CLI
will emit a somewhat nicer error message when parsing fails (indicating
which argument the error occurred while parsing, including the invalid
value, and suggesting the help text). On the other hand, when we accept
the argument as a `String`, `clap` doesn't add that useful context to
the error message.

This branch adds a `ScopedV6Addr` newtype, consisting of an `Ipv6Addr`
and a scope identifier, and adds a `FromStr` implementation for that
type. Now, we can use `ScopedV6Addr` as the type of the CLI argument,
and `clap` performs the parsing for us, getting us somewhat nicer error
messages. Compare the error output from master:

```console
$ cargo run -- --ip ::%fakeiface0 probe
    Finished dev [unoptimized + debuginfo] target(s) in 0.13s
     Running `target/debug/humility --ip '::%fakeiface0' probe`
humility probe failed: Could not find interface for fakeiface0

$ cargo run -- --ip :: probe
    Finished dev [unoptimized + debuginfo] target(s) in 0.13s
     Running `target/debug/humility --ip '::' probe`
humility probe failed: Missing scope id in IP (e.g. '%en0')

$ cargo run -- --ip barf%eth0 probe
    Finished dev [unoptimized + debuginfo] target(s) in 0.13s
     Running `target/debug/humility --ip 'barf%eth0' probe`
humility probe failed: invalid Zip archive: Invalid zip header
```

with the error output on this branch:

```console
$ cargo run -- --ip :: probe
    Finished dev [unoptimized + debuginfo] target(s) in 0.14s
     Running `target/debug/humility --ip '::' probe`
error: Invalid value "::" for '--ip <IP>': missing scope ID (e.g. "%en0") in IPv6 address

For more information try --help

$ cargo run -- --ip ::%fakeiface0 probe
    Finished dev [unoptimized + debuginfo] target(s) in 8.45s
     Running `target/debug/humility --ip '::%fakeiface0' probe`
error: Invalid value "::%fakeiface0" for '--ip <IP>': Could not find interface for fakeiface0

For more information try --help

$ cargo run -- --ip barf%eth0 probe
    Finished dev [unoptimized + debuginfo] target(s) in 12.87s
     Running `target/debug/humility --ip 'barf%eth0' probe`
error: Invalid value "barf%eth0" for '--ip <IP>': "barf" is not a valid IPv6 address

For more information try --help
```

Additionally, using the `ScopedIpv6Addr` type allows us to provide a
`fmt::Display` implementation that combines the IP address and scope ID
when formatting. This lets us get rid of a little bit of repeated logic
for formatting an address with the delimiter, which seems kinda nice to
me.

Because `humility rpc` accepts a list of addresses or a `-i <INTERFACE>`
(with `--listen`), we still need to use `decode_iface` directly when
called with `-i`.