Auto merge of #2196 - matklad:metadata2, r=alexcrichton

Most of the work was done by @dan-t in #1225 and by @winger in #1434

Fixes #2193

I failed to properly rebase previous attempts so I just salvaged this from bits and pieces.

@alexcrichton are you sure that the default format should be TOML? I think that TOML is more suitable for humans, and JSON is better (at the moment at least) for tools. Maybe we should default to ~~TOML~~ JSON?

diff --cc src/bin/cargo.rs
Simple merge

diff --cc tests/tests.rs
Simple merge