Update renamer errors to clarify some funny situations

This tries to address #125 by making the errors from the renamer a little bit
more clear.

Squashed commit of the following:

commit 8afd3d7961b58df042fe801c3c5e1b9787f813bc
Author: Trevor Elliott <trevor@galois.com>
Date:   Wed Dec 3 19:33:59 2014 -0800

    Update tests for new renamer errors

commit 7cac01836d8943cf3b08d6715ac328e3b6658cef
Author: Trevor Elliott <trevor@galois.com>
Date:   Wed Dec 3 19:33:49 2014 -0800

    Add `at` on errors and warnings to be more consistent

commit 308908ba318a4cdc839710f66f1a487543f8c07e
Author: Trevor Elliott <trevor@galois.com>
Date:   Wed Dec 3 19:06:57 2014 -0800

    More consistent renamer warnings

commit be8100a78e9eaba6d554591121c24ed5dcd3c780
Author: Trevor Elliott <trevor@galois.com>
Date:   Wed Dec 3 18:56:53 2014 -0800

    More consistent error formatting from the renamer

commit 26c45c3b51e0bdbcf6a1431cab8e1eb8760ea0bb
Author: Trevor Elliott <trevor@galois.com>
Date:   Wed Dec 3 18:56:36 2014 -0800

    Remove an un-triggerable error

commit ccdb93e036ba1e111ccd977c8b3b35523f3c1bf0
Author: Trevor Elliott <trevor@galois.com>
Date:   Wed Dec 3 16:38:44 2014 -0800

    Try to give better errors for unbound identifiers

commit eb5784145985bb55c761088eaba27c67d08c1326
Author: Trevor Elliott <trevor@galois.com>
Date:   Wed Dec 3 16:38:23 2014 -0800

    Remove old TODOs about located errors

commit b984bb5f451f3aa7b4fc8f15167483c5142ee9a3
Author: Trevor Elliott <trevor@galois.com>
Date:   Wed Dec 3 14:37:34 2014 -0800

    Differentiate missing type and expression symbols

commit b9e6f13856db6765dced3cb9565cdc8387a7976d
Author: Trevor Elliott <trevor@galois.com>
Date:   Wed Dec 3 14:36:52 2014 -0800

    Remove a shadowing warning