Improve typechecking of let-bindings

This major commit was initially triggered by #11339, but it spiraled
into a major review of the way in which type signatures for bindings
are handled, especially partial type signatures.  On the way I fixed a
number of other bugs, namely
   #12069
   #12033
   #11700
   #11339
   #11670

The main change is that I completely reorganised the way in which type
signatures in bindings are handled. The new story is in TcSigs
Note [Overview of type signatures].  Some specific:

* Changes in the data types for signatures in TcRnTypes:
  TcIdSigInfo and new TcIdSigInst

* New module TcSigs deals with typechecking type signatures
  and pragmas. It contains code mostly moved from TcBinds,
  which is already too big

* HsTypes: I swapped the nesting of HsWildCardBndrs
  and HsImplicitBndsrs, so that the wildcards are on the
  oustide not the insidde in a LHsSigWcType.  This is just
  a matter of convenient, nothing deep.

There are a host of other changes as knock-on effects, and
it all took FAR longer than I anticipated :-).  But it is
a significant improvement, I think.

Lots of error messages changed slightly, some just variants but
some modest improvements.

New tests

* typecheck/should_compile
    * SigTyVars: a scoped-tyvar test
    * ExPat, ExPatFail: existential pattern bindings
    * T12069
    * T11700
    * T11339

* partial-sigs/should_compile
    * T12033
    * T11339a
    * T11670

One thing to check:

* Small change to output from ghc-api/landmines.
  Need to check with Alan Zimmerman

1 files changed, 32 insertions, 36 deletions

diff --git a/testsuite/tests/partial-sigs/should_fail/T10615.stderr b/testsuite/tests/partial-sigs/should_fail/T10615.stderr
index 842b2eb10a..9e46dd20d1 100644
--- a/testsuite/tests/partial-sigs/should_fail/T10615.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/T10615.stderr
@@ -1,36 +1,32 @@
-
-T10615.hs:4:7: error:
-    • Found type wildcard ‘_’ standing for ‘a1’
-      Where: ‘a1’ is an ambiguous type variable
-      To use the inferred type, enable PartialTypeSignatures
-    • In the type signature:
-        f1 :: _ -> f
-    • Relevant bindings include f1 :: a1 -> f (bound at T10615.hs:5:1)
-
-T10615.hs:5:6: error:
-    • Couldn't match type ‘f’ with ‘b1 -> a1’
-      ‘f’ is a rigid type variable bound by
-        the inferred type of f1 :: a1 -> f at T10615.hs:4:7
-      Expected type: a1 -> f
-        Actual type: a1 -> b1 -> a1
-    • In the expression: const
-      In an equation for ‘f1’: f1 = const
-    • Relevant bindings include f1 :: a1 -> f (bound at T10615.hs:5:1)
-
-T10615.hs:7:7: error:
-    • Found type wildcard ‘_’ standing for ‘a0’
-      Where: ‘a0’ is an ambiguous type variable
-      To use the inferred type, enable PartialTypeSignatures
-    • In the type signature:
-        f2 :: _ -> _f
-    • Relevant bindings include f2 :: a0 -> _f (bound at T10615.hs:8:1)
-
-T10615.hs:8:6: error:
-    • Couldn't match type ‘_f’ with ‘b0 -> a0’
-      ‘_f’ is a rigid type variable bound by
-        the inferred type of f2 :: a0 -> _f at T10615.hs:7:7
-      Expected type: a0 -> _f
-        Actual type: a0 -> b0 -> a0
-    • In the expression: const
-      In an equation for ‘f2’: f2 = const
-    • Relevant bindings include f2 :: a0 -> _f (bound at T10615.hs:8:1)
+

+T10615.hs:4:7: error:

+    • Found type wildcard ‘_’ standing for ‘a1’

+      Where: ‘a1’ is an ambiguous type variable

+      To use the inferred type, enable PartialTypeSignatures

+    • In the type signature: f1 :: _ -> f

+

+T10615.hs:5:6: error:

+    • Couldn't match type ‘f’ with ‘b1 -> a1’

+      ‘f’ is a rigid type variable bound by

+        the inferred type of f1 :: a1 -> f at T10615.hs:5:1-10

+      Expected type: a1 -> f

+        Actual type: a1 -> b1 -> a1

+    • In the expression: const

+      In an equation for ‘f1’: f1 = const

+    • Relevant bindings include f1 :: a1 -> f (bound at T10615.hs:5:1)

+

+T10615.hs:7:7: error:

+    • Found type wildcard ‘_’ standing for ‘a0’

+      Where: ‘a0’ is an ambiguous type variable

+      To use the inferred type, enable PartialTypeSignatures

+    • In the type signature: f2 :: _ -> _f

+

+T10615.hs:8:6: error:

+    • Couldn't match type ‘_f’ with ‘b0 -> a0’

+      ‘_f’ is a rigid type variable bound by

+        the inferred type of f2 :: a0 -> _f at T10615.hs:8:1-10

+      Expected type: a0 -> _f

+        Actual type: a0 -> b0 -> a0

+    • In the expression: const

+      In an equation for ‘f2’: f2 = const

+    • Relevant bindings include f2 :: a0 -> _f (bound at T10615.hs:8:1)