Support a backtrace

[shepmaster]

People like backtraces.

My idea is to support a field called backtrace. If it's present, it will be automatically constructed when From::from or fail is called.

One trick will be to decide how best to have forwards-compatibility. Eventually, the standard library will have such a type and it will accessible via the Error trait, but it's not there today. We'd prefer to be able to be able to seamlessly upgrade.

My thought is to have a newtype for the backtrace crate that doesn't expose much in the way of an API. We can then add an inherent method on the Error enum with the proposed API for std::error::Error.

Things to check / consider:

• What happens when std::error::Error gains the method; will that cause conflicts?
• Do we need multiple ways to specify what the backtrace is? (see Support additional ways of specifying the error source or backtrace #10)

[shepmaster]

• What happens when std::error::Error gains the method; will that cause conflicts?

I decided to create a parallel ErrorCompat trait and encourage people to use the methods explicitly.

[shepmaster]

• Do we need multiple ways to specify what the backtrace is?

Postponing this.