The implicit parameter isn't actually very relevant to the CallStack
machinery, so we hide the implementation details behind a constraint
alias

```
type HasCallStack = (?callStack :: CallStack)
```

This has a few benefits:

1. No need to enable `ImplicitParams` in user code.
2. No need to remember the `?callStack` naming convention.
3. Gives us the option to change the implementation details in the
future with less user-land breakage.

The revised `CallStack` API is exported from `GHC.Stack` and makes no
mention of the implicit parameter.

Test Plan: ./validate

Reviewers: simonpj, austin, hvr, bgamari

Reviewed By: simonpj, bgamari

Subscribers: thomie

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D1818