Minor cleanup on dependency insertion and loading

[sgrif]

I'm going through some random code as I'm looking to point to this repo
more in Diesel's guides for some actual examples. My goal was actually
to do the following:

• #[derive(Insertable)] on upload::CrateDependency and use that
  directly
• #[derive(Queryable)] on git::Dependency and construct it from a
  returning clause

However, doing the first piece would require skipping the crate_name
field or pulling it out of that struct, and the second piece requires
some way to tell Diesel that a subselect returns a single row (which is
not hard to do, but is not code that should live in this repo)

Either way, while I was experimenting I did a bit of cleanup here which
I think is worthwhile on its own.

• Removed an Insertable struct in favor of tuple inserts, since
  Insertable is really only mean to be used when your input comes from
  another source (like Deserialize).
• Made some defaults explicit in the tests. It felt sketchy having the
  default impl that was only used in tests. I thought about pushing
  these defaults into the database, but that felt sketchy for the same
  reason
• Removed a potential panic
• Removed a few unneccessary iterations/allocations over the features
  vec.

[jtgeibel]

Looks good to me! I'm not sure why resolving through the web interface didn't work the first time.

bors: r+

[bors-voyager]

Build succeeded

• continuous-integration/travis-ci/push