Do eager instantation in terms

This patch implements eager instantiation, a small but critical change
to the type inference engine, #17173.  The main change is this:

  When inferring types, always return an instantiated type
  (for now, deeply instantiated; in future shallowly instantiated)

There is more discussion in
https://www.tweag.io/posts/2020-04-02-lazy-eager-instantiation.html

There is quite a bit of refactoring in this patch:

* The ir_inst field of GHC.Tc.Utils.TcType.InferResultk
  has entirely gone.  So tcInferInst and tcInferNoInst have collapsed
  into tcInfer.

* Type inference of applications, via tcInferApp and
  tcInferAppHead, are substantially refactored, preparing
  the way for Quick Look impredicativity.

* New pure function GHC.Tc.Gen.Expr.collectHsArgs and applyHsArgs
  are beatifully dual.  We can see the zipper!

* GHC.Tc.Gen.Expr.tcArgs is now much nicer; no longer needs to return
  a wrapper

* In HsExpr, HsTypeApp now contains the the actual type argument,
  and is used in desugaring, rather than putting it in a mysterious
  wrapper.

* I struggled a bit with good error reporting in
  Unify.matchActualFunTysPart. It's a little bit simpler than before,
  but still not great.

Some smaller things

* Rename tcPolyExpr --> tcCheckExpr
         tcMonoExpr --> tcLExpr
* tcPatSig moves from GHC.Tc.Gen.HsType to GHC.Tc.Gen.Pat

Metric Decrease:
    T9961

Reduction of 1.6% in comiler allocation on T9961, I think.

3 files changed, 20 insertions, 17 deletions

diff --git a/testsuite/tests/ghci/scripts/Defer02.stderr b/testsuite/tests/ghci/scripts/Defer02.stderr
index 63dbc9b042..0defd52b38 100644
--- a/testsuite/tests/ghci/scripts/Defer02.stderr
+++ b/testsuite/tests/ghci/scripts/Defer02.stderr
@@ -26,8 +26,8 @@ Defer01.hs:25:4: warning: [-Winaccessible-code (in -Wdefault)]
 
 Defer01.hs:31:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
     • Couldn't match expected type ‘Char -> t’ with actual type ‘Char’
-    • The function ‘e’ is applied to one argument,
-      but its type ‘Char’ has none
+    • The function ‘e’ is applied to one value argument,
+        but its type ‘Char’ has none
       In the expression: e 'q'
       In an equation for ‘f’: f = e 'q'
     • Relevant bindings include f :: t (bound at Defer01.hs:31:1)
@@ -79,7 +79,7 @@ Defer01.hs:50:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
     • In the expression: 'p'
       In an equation for ‘a’: a = 'p'
 (deferred type error)
-*** Exception: Defer01.hs:18:7: error:
+*** Exception: Defer01.hs:18:9: error:
     • No instance for (Eq B) arising from a use of ‘==’
     • In the expression: x == x
       In an equation for ‘b’: b x = x == x
@@ -100,8 +100,8 @@ Defer01.hs:50:5: warning: [-Wdeferred-type-errors (in -Wdefault)]
 (deferred type error)
 *** Exception: Defer01.hs:31:5: error:
     • Couldn't match expected type ‘Char -> t’ with actual type ‘Char’
-    • The function ‘e’ is applied to one argument,
-      but its type ‘Char’ has none
+    • The function ‘e’ is applied to one value argument,
+        but its type ‘Char’ has none
       In the expression: e 'q'
       In an equation for ‘f’: f = e 'q'
     • Relevant bindings include f :: t (bound at Defer01.hs:31:1)
diff --git a/testsuite/tests/ghci/scripts/T10963.stderr b/testsuite/tests/ghci/scripts/T10963.stderr
index 2efd138be8..aa081391c4 100644
--- a/testsuite/tests/ghci/scripts/T10963.stderr
+++ b/testsuite/tests/ghci/scripts/T10963.stderr
@@ -1,12 +1,13 @@
 
 <interactive>:1:1: error:
-    Ambiguous type variable ‘a0’ arising from a use of ‘foo’
-    prevents the constraint ‘(Num a0)’ from being solved.
-    Probable fix: use a type annotation to specify what ‘a0’ should be.
-    These potential instances exist:
-      instance Num Integer -- Defined in ‘GHC.Num’
-      instance Num Double -- Defined in ‘GHC.Float’
-      instance Num Float -- Defined in ‘GHC.Float’
-      ...plus two others
-      ...plus 8 instances involving out-of-scope types
-      (use -fprint-potential-instances to see them all)
+    • Ambiguous type variable ‘a0’ arising from a use of ‘foo’
+      prevents the constraint ‘(Num a0)’ from being solved.
+      Probable fix: use a type annotation to specify what ‘a0’ should be.
+      These potential instances exist:
+        instance Num Integer -- Defined in ‘GHC.Num’
+        instance Num Double -- Defined in ‘GHC.Float’
+        instance Num Float -- Defined in ‘GHC.Float’
+        ...plus two others
+        ...plus 8 instances involving out-of-scope types
+        (use -fprint-potential-instances to see them all)
+    • In the expression: foo
diff --git a/testsuite/tests/ghci/scripts/ghci047.stderr b/testsuite/tests/ghci/scripts/ghci047.stderr
index 86130800b0..90a9bb7c60 100644
--- a/testsuite/tests/ghci/scripts/ghci047.stderr
+++ b/testsuite/tests/ghci/scripts/ghci047.stderr
@@ -2,11 +2,13 @@
 <interactive>:40:1: error:
     • Couldn't match type ‘HFalse’ with ‘HTrue’
         arising from a use of ‘f’
-    • In the expression: f $ Baz 'a'
+    • In the first argument of ‘($)’, namely ‘f’
+      In the expression: f $ Baz 'a'
       In an equation for ‘it’: it = f $ Baz 'a'
 
 <interactive>:41:1: error:
     • Couldn't match type ‘HFalse’ with ‘HTrue’
         arising from a use of ‘f’
-    • In the expression: f $ Quz
+    • In the first argument of ‘($)’, namely ‘f’
+      In the expression: f $ Quz
       In an equation for ‘it’: it = f $ Quz