Implement -XStaticValues

Summary:
As proposed in [1], this extension introduces a new syntactic form
`static e`, where `e :: a` can be any closed expression. The static form
produces a value of type `StaticPtr a`, which works as a reference that
programs can "dereference" to get the value of `e` back. References are
like `Ptr`s, except that they are stable across invocations of a
program.

The relevant wiki pages are [2, 3], which describe the motivation/ideas
and implementation plan respectively.

[1] Jeff Epstein, Andrew P. Black, and Simon Peyton-Jones. Towards
Haskell in the cloud. SIGPLAN Not., 46(12):118–129, September 2011. ISSN
0362-1340.
[2] https://ghc.haskell.org/trac/ghc/wiki/StaticPointers
[3] https://ghc.haskell.org/trac/ghc/wiki/StaticPointers/ImplementationPlan

Authored-by: Facundo Domínguez <facundo.dominguez@tweag.io>
Authored-by: Mathieu Boespflug <m@tweag.io>
Authored-by: Alexander Vershilov <alexander.vershilov@tweag.io>

Test Plan: `./validate`

Reviewers: hvr, simonmar, simonpj, austin

Reviewed By: simonpj, austin

Subscribers: qnikst, bgamari, mboes, carter, thomie, goldfire

Differential Revision: https://phabricator.haskell.org/D550

GHC Trac Issues: #7015
fix misleading error message regarding function arity

Summary:
The error reports something like:

  The function ‘f’ is applied to three arguments,
  but its type ‘Int -> Float -> Char -> Bool’ has only three

The original type was "Monad m => Int -> Float -> m Bool", but
"m" was unified with "-> Char".

Now it looks better:

  The function ‘f’ is applied to three arguments,
  its type is ‘Int -> Float -> m0 Bool’,
  it is specialized to ‘Int -> Float -> Char -> Bool’

Test Plan: T9605

Reviewers: simonpj, austin

Reviewed By: austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D556

GHC Trac Issues: #9605
Document splitAt deviation from the Report

Summary:
`splitAt` is stricter than the Report specifies, so we should
say so.

Reviewers: hvr, austin

Reviewed By: austin

Subscribers: carter, thomie

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D562

GHC Trac Issues: #9870
Link pre-ARMv6 spinlocks into all RTS variants

Summary:
For compatibility with ARM machines from pre v6, the RTS provides
implementations of certain atomic operations. Previously, these
were only included in the threaded RTS.

But ghc (the library) contains the code in compiler/cbits/genSym.c, which
uses these operations if there is more than one capability. But there is only
one libHSghc, so the linker wants to resolve these symbols in every case.

By providing these operations in all RTSs, the linker is happy. The only
downside is a small amount of dead code in the non-threaded RTS on old ARM
machines.

Test Plan: It helped here.

Reviewers: bgamari, austin

Reviewed By: bgamari, austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D564

GHC Trac Issues: #8951