* Introduce Stacktrace and Frame
This PR is a continuation of a series aimed at exposing the stack trace
information embedded in each error value. The secondary effect is to
deprecated the `Fprintf` helper.
Taking cues from from @chrishines' `stack` package this PR introduces a
new interface `Stacktrace() []Frame` and a `Frame` type, similar in
function to the `runtime.Frame` type (although lacking its iterator
type). Each `Frame` implemnts `fmt.Formatter` allowing it to print
itself.
The older `Location` interface is still supported but also deprecated.
