Export scanl' from Data.OldList and Data.List

Summary: Fixes #9368

Reviewers: nomeata, hvr, ekmett, austin

Reviewed By: ekmett, austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D498

GHC Trac Issues: #9368
Add flag `-fwarn-missing-exported-sigs`

Summary: add `-fwarn-missing-exported-sigs` to only warn about missing signatures if the name is exported

Test Plan: validate, see testsuite/tests/warnings/should_compile/T2526.hs

Reviewers: ezyang, austin, thomie

Reviewed By: austin, thomie

Subscribers: ezyang, thomie, carter

Differential Revision: https://phabricator.haskell.org/D482

GHC Trac Issues: #2526

Conflicts:
	docs/users_guide/7.10.1-notes.xml
Implement #5462 (deriving clause for arbitrary classes)

Summary: (this has been submitted on behalf on @dreixel)

Reviewers: simonpj, hvr, austin

Reviewed By: simonpj, austin

Subscribers: goldfire, thomie, carter, dreixel

Differential Revision: https://phabricator.haskell.org/D476

GHC Trac Issues: #5462