Use cargo metadata to find cargo manifest

[jDomantas]

Use cargo metadata itself to find Cargo.toml instead of manually walking up the directory tree.

There is a small difference in behavior: now you can't run bacon path/to/Cargo.toml (and instead you have to do bacon path/to). It seems that previous behavior was accidentally allowed as it would treat the path like a directory and first check path/to/Cargo.toml/Cargo.toml for existence, followed by path/to/Cargo.toml.

The couple of expect()s should not fail unless I misunderstood cargo metadata docs.

Fixes #59.

[Canop]

I'm OK for not allowing a path to the Cargo.toml file but there's a regression IMO.

Old and new behaviors when launched without path as argument and not in a Rust project:

Can you please restore the old error message ? The new one doesn't hint at the possibility to give a path as argument.

[jDomantas]

Restored the old message. Sadly cargo metadata does not emit structured error data, so I hope that string matching on stderr is good enough.

[Canop]

This is probably the right approach.
I'll have a lot of checks and tests to do before I can merge that, though. People with strangely structured workspaces are encouraged to test and tell me how it is.

Regarding the errors in metadata, a PR there would probably be a good idea too (it's still 0.x so this kind of breaking change should be OK).