chain_display with alternate format for linebreaks and backtraces

This is a riff on #233, prototype based on discussions of that PR.

• Uses chain_display() -> ChainDisplay as rename of display() -> FailDisplay

• Adds a convenient top level failure::chain_display function, which simplifies usage (see examples/simple changes).

• Uses single line format for "{}" and multiple lines with backtraces for (alternate == true) "{:#}".

• Propagates alternate format selection to Fail types Display::fmt.

• Extends examples/simple.rs with all output variants. (There is not unit or doctests yet to riff on.)

• Adds a example/custom.rs which might demonstrate a typical double (but not particularly harmful) output of an inner error, based on current usage.

As another improvement (beyond #233) add (as of 37dafed) a Debug implementation for ChainDisplay as well.

Formal rationale: As Fail requires Debug, so should ChainDisplay be Debug.

Practical reasoning: Users are likely to want this when println! debugging applications.

Like with the alternate Display format, Debug formatting is propagated to each Fail in the chain.