Haddock interfaces produced from `.hi` files

= Summary

This is a large architectural change to how Haddock interfaces are
computed. This is a rebased variant of the second half of Simon Jakobi's
(@sjakobi) Hi Haddock GSOC project.

The idea is to work over top of the contents of `.hi` files
instead of over the internal compiler representations of ASTs.
This has a wide array of benefits, but two main ones are:

  * Haddock doesn't necessarily need to recompile modules to generate
    docs - if the right `.hi` files already exist, it'll automatically
    load them. Even better: this is nothing more that the existing
    recompilation avoidance logic in GHC.

  * Haddock no longer operates over concrete syntax - all declarations
    in the final docs are the result of reifying a `TyThing`. This means
    that the docs can be somewhat agnostic of users' particular choice
    to add certain kind annotations, use certain  special syntax, etc.

== Before

Very roughly the previous way this worked was:

  1. `depanal` is used to figure out the dependency order in which
     modules should be loaded

  2. `parseModule` and `typecheckModule` are used to get a
     `TypecheckedModule` for every input module

  3. The parsed and renamed sources in the typechecked module are
     traversed in various ways accumulating all the right info

== After

Now, the process

  1. `GhcMake.load'` is used to load `.hi`-files into GHC

  2. `loadSysInterface` is used to lookup the `ModIface` associated with
     each module