Summary:
This refactors (>>)/(*>)/return/pure methods into normalform.

The redundant explicit `return` method definitions are dropped
altogether.

This results in measurable runtime improvements in nofib of up
to -20% runtime (geometric mean: -7%) in my measurements.

Reviewers: quchen, austin, alanz, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1316

Among doing other things, Phab:D201 (bc2289e13d9586be087bd8136943dc35a0130c88)
tried to improve the error messages thrown by the parser. For example a missing
else clause now prints "parse error in if statement: else clause empty" instead
of "parse error (possibly incorrect indentation or mismatched brackets)".

Some error messages got much worse however (see tests), and the result seems to
be a net negative. Although not entirely satisfactory, this commits therefore
reverts those parser changes.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D1309

GHC Trac Issues: #10498

For example

```
pattern head `Cons` tail = head : tail
```

Reviewed By: goldfire, austin

Differential Revision: https://phabricator.haskell.org/D1295

GHC Trac Issues: #10747

Improved error messages are only printed when the old message would be
"No instance for...", since they're not as helpful for "Could not deduce..."

No special test case as error messages are tested by other tests already.

Signed-off-by: David Kraeutmann <kane@kane.cx>

Reviewed By: austin, goldfire

Differential Revision: https://phabricator.haskell.org/D1182

GHC Trac Issues: #10733

Fixes #10571.

Apparently some Clang 3.6 expects these to be sorted.

Patch due to Peter Wortmann.

Fixes #10687.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Triggered by investigations around Trac 10845