Summary:
Ties up loose ends from D1894.

GHC 7.10.2 and 7.10.3 featured a `Show` instance for `CallStack`, but since it
was derived, it broke encapsulation. This adds a `Show` instance which displays
the `CallStack` as if it were a `[(String, SrcLoc)]`.

To ensure that the output of `Show` is technically a valid Haskell term, we
also add a corresponding `IsList CallStack` instance.

Reviewers: gridaphobe, austin, hvr, bgamari

Reviewed By: gridaphobe, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1903