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This patch adds a go-multiaddr CLI to aid in testing this
implementation of multiaddr. It takes a multiaddr in string or
packed form as input, and prints detailed information about the
multiaddr. This tool can be useful beyond testing, of course.

Another addition is a Makefile target for running the new
multiaddr conformance test suite. This test suite lives at
https://github.com/multiformats/multiaddr and is fetched to be
run against our new go-multiaddr CLI. This target is to be run in CI

Neither the test suite nor the CLI are complete yet.

Currently the output looks like this:

```
> go run ./multiaddr /ip4/192.0.2.42/tcp/443 | jq .
{
  "string": "/ip4/192.0.2.42/tcp/443",
  "stringSize": "23",
  "packed": "0x04c000022a0601bb",
  "packedSize": "8",
  "components": [
    {
      "string": "/ip4/192.0.2.42",
      "stringSize": "15",
      "packed": "0x04c000022a",
      "packedSize": "5",
      "value": "192.0.2.42",
      "rawValue": "0xc000022a",
      "valueSize": "4",
      "protocol": "ip4",
      "codec": "4",
      "uvarint": "0x04",
      "lengthPrefix": ""
    },
    {
      "string": "/tcp/443",
      "stringSize": "8",
      "packed": "0x0601bb",
      "packedSize": "3",
      "value": "443",
      "rawValue": "0x01bb",
      "valueSize": "2",
      "protocol": "tcp",
      "codec": "6",
      "uvarint": "0x06",
      "lengthPrefix": ""
    }
  ]
}
```

And the Makefile target:

```
> make conformance
go get -d -v .
go build -o tmp/multiaddr/test/go-multiaddr ./multiaddr
cd tmp/multiaddr/test && MULTIADDR_BIN="./go-multiaddr" go test -v
=== RUN   TestGodog
MULTIADDR_BIN="./go-multiaddr"
Feature: Multiaddr

  Scenario: Banana                              # multiaddr.feature:3
    Given the multiaddr /ip4/192.0.2.42/tcp/443 # main_test.go:81 -> github.com/multiformats/multiaddr/test_test.theMultiaddr
    Then the packed form is 0x04c000022a0601bb  # main_test.go:98 -> github.com/multiformats/multiaddr/test_test.thePackedFormIs
    And the packed size is 8 bytes              # main_test.go:105 -> github.com/multiformats/multiaddr/test_test.thePackedSizeIs
    And the components are:                     # main_test.go:126 -> github.com/multiformats/multiaddr/test_test.theComponentsAre
      | string          | stringSize | packed       | packedSize | value      | valueSize | protocol | codec | uvarint | lengthPrefix | rawValue   |
      | /ip4/192.0.2.42 | 15         | 0x04c000022a | 5          | 192.0.2.42 | 4         | ip4      | 4     | 0x04    |              | 0xc000022a |
      | /tcp/443        | 8          | 0x0601bb     | 3          | 443        | 2         | tcp      | 6     | 0x06    |              | 0x01bb     |

  Scenario: Banana #2                               # multiaddr.feature:12
    Given the multiaddr 0x04c000022a0601bb          # main_test.go:81 -> github.com/multiformats/multiaddr/test_test.theMultiaddr
    Then the string form is /ip4/192.0.2.42/tcp/443 # main_test.go:112 -> github.com/multiformats/multiaddr/test_test.theStringFormIs
    And the string size is 23 bytes                 # main_test.go:119 -> github.com/multiformats/multiaddr/test_test.theStringSizeIs
    And the components are:                         # main_test.go:126 -> github.com/multiformats/multiaddr/test_test.theComponentsAre
      | string          | stringSize | packed       | packedSize | value      | valueSize | protocol | codec | uvarint | lengthPrefix | rawValue   |
      | /ip4/192.0.2.42 | 15         | 0x04c000022a | 5          | 192.0.2.42 | 4         | ip4      | 4     | 0x04    |              | 0xc000022a |
      | /tcp/443        | 8          | 0x0601bb     | 3          | 443        | 2         | tcp      | 6     | 0x06    |              | 0x01bb     |

2 scenarios (2 passed)
8 steps (8 passed)
3.187755ms
--- PASS: TestGodog (0.00s)
PASS
ok  	github.com/multiformats/multiaddr/test	0.012s
```