Linear types (#15981)

This is the first step towards implementation of the linear types proposal
(https://github.com/ghc-proposals/ghc-proposals/pull/111).

It features

* A language extension -XLinearTypes
* Syntax for linear functions in the surface language
* Linearity checking in Core Lint, enabled with -dlinear-core-lint
* Core-to-core passes are mostly compatible with linearity
* Fields in a data type can be linear or unrestricted; linear fields
  have multiplicity-polymorphic constructors.
  If -XLinearTypes is disabled, the GADT syntax defaults to linear fields

The following items are not yet supported:

* a # m -> b syntax (only prefix FUN is supported for now)
* Full multiplicity inference (multiplicities are really only checked)
* Decent linearity error messages
* Linear let, where, and case expressions in the surface language
  (each of these currently introduce the unrestricted variant)
* Multiplicity-parametric fields
* Syntax for annotating lambda-bound or let-bound with a multiplicity
* Syntax for non-linear/multiple-field-multiplicity records
* Linear projections for records with a single linear field
* Linear pattern synonyms
* Multiplicity coercions (test LinearPolyType)

A high-level description can be found at
https://ghc.haskell.org/trac/ghc/wiki/LinearTypes/Implementation
Following the link above you will find a description of the changes made to Core.
This commit has been authored by

* Richard Eisenberg
* Krzysztof Gogolewski
* Matthew Pickering
* Arnaud Spiwack

With contributions from:

* Mark Barbone
* Alexander Vershilov

Updates haddock submodule.

1 files changed, 29 insertions, 0 deletions

diff --git a/testsuite/tests/linear/should_compile/T1735Min.hs b/testsuite/tests/linear/should_compile/T1735Min.hs
new file mode 100644
index 0000000000..8800272328
--- /dev/null
+++ b/testsuite/tests/linear/should_compile/T1735Min.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE LinearTypes #-}
+{-# LANGUAGE UndecidableInstances, Rank2Types,
+    KindSignatures, EmptyDataDecls, MultiParamTypeClasses, CPP #-}
+
+module T1735_Help.Basics where
+
+data Proxy a = Proxy
+
+class Data ctx a where
+     gunfold :: Proxy ctx
+             -> (forall b r. Data ctx b => c (b -> r) -> c r)
+             -> (forall r. r -> c r)
+             -> Constr
+             -> c a
+
+
+newtype ID x = ID { unID :: x }
+
+fromConstrB :: Data ctx a
+            => Proxy ctx
+            -> (forall b. Data ctx b => b)
+            -> Constr
+            -> a
+fromConstrB ctx f = unID . gunfold ctx k z
+ where
+  k c = ID (unID c f)
+  z = ID
+
+data Constr = Constr