Auto merge of #2219 - matklad:encodable-audit, r=alexcrichton

Auto merge of #2219 - matklad:encodable-audit, r=alexcrichton

@alexcrichton another preparation PR for #2196

I've removed obscure `metadata` field from `Target`. It is a breaking change (for read-manifest), but the field seemed cryptic, useless and untested :)

`Target` has a bunch of boolean fields:

```
    tested: bool,
    benched: bool,
    doc: bool,
    doctest: bool,
    harness: bool, // whether to use the test harness (--test)
    for_host: bool,
```

I guess they should not be included in serialized representation?

I will push commits for other `Encodable`s here.