Impl HttpTryFrom<Parts> for Uri

[carllerche]

This allows Parts to be used with the builder fns.

[seanmonstar]

On mobile, so can't see very easily: is it possible to construct a Parts that isn't a valid Uri? Like if you set authority and path, but no scheme?

This impl looks fine.

[carllerche]

There currently is an Into implementation, so if there is it would be incorrect...

That said, it might be possible, so that should be audited. I will open an issue to check.

[carllerche]

I created #96 to revisit the Into implementation. If there are error cases, they should be reflected in the HttpTryInto impl as part of resolving #96.