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, 38 insertions, 44 deletions

diff --git a/testsuite/tests/partial-sigs/should_compile/T11192.stderr b/testsuite/tests/partial-sigs/should_compile/T11192.stderr
index 558097ca2b..c2a9db5a96 100644
--- a/testsuite/tests/partial-sigs/should_compile/T11192.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/T11192.stderr
@@ -1,44 +1,38 @@
-
-T11192.hs:7:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘Int -> t -> t’
-      Where: ‘t’ is a rigid type variable bound by
-               the inferred type of go :: Int -> t -> t at T11192.hs:8:8
-    • In the type signature:
-        go :: _
-      In the expression:
-        let
-          go :: _
-          go 0 a = a
-        in go (0 :: Int) undefined
-      In an equation for ‘fails’:
-          fails
-            = let
-                go :: _
-                go 0 a = a
-              in go (0 :: Int) undefined
-    • Relevant bindings include
-        go :: Int -> t -> t (bound at T11192.hs:8:8)
-        fails :: a (bound at T11192.hs:6:1)
-
-T11192.hs:13:14: warning: [-Wpartial-type-signatures (in -Wdefault)]
-    • Found type wildcard ‘_’ standing for ‘t1 -> t -> t’
-      Where: ‘t’ is a rigid type variable bound by
-               the inferred type of go :: t1 -> t -> t at T11192.hs:14:8
-             ‘t1’ is a rigid type variable bound by
-               the inferred type of go :: t1 -> t -> t at T11192.hs:14:8
-    • In the type signature:
-        go :: _
-      In the expression:
-        let
-          go :: _
-          go _ a = a
-        in go (0 :: Int) undefined
-      In an equation for ‘succeeds’:
-          succeeds
-            = let
-                go :: _
-                go _ a = a
-              in go (0 :: Int) undefined
-    • Relevant bindings include
-        go :: t1 -> t -> t (bound at T11192.hs:14:8)
-        succeeds :: a (bound at T11192.hs:12:1)
+

+T11192.hs:7:14: warning: [-Wpartial-type-signatures (in -Wdefault)]

+    • Found type wildcard ‘_’ standing for ‘Int -> t -> t’

+      Where: ‘t’ is a rigid type variable bound by

+               the inferred type of go :: Int -> t -> t at T11192.hs:8:8-17

+    • In the type signature: go :: _

+      In the expression:

+        let

+          go :: _

+          go 0 a = a

+        in go (0 :: Int) undefined

+      In an equation for ‘fails’:

+          fails

+            = let

+                go :: _

+                go 0 a = a

+              in go (0 :: Int) undefined

+    • Relevant bindings include fails :: a (bound at T11192.hs:6:1)

+

+T11192.hs:13:14: warning: [-Wpartial-type-signatures (in -Wdefault)]

+    • Found type wildcard ‘_’ standing for ‘t1 -> t -> t’

+      Where: ‘t’ is a rigid type variable bound by

+               the inferred type of go :: t1 -> t -> t at T11192.hs:14:8-17

+             ‘t1’ is a rigid type variable bound by

+               the inferred type of go :: t1 -> t -> t at T11192.hs:14:8-17

+    • In the type signature: go :: _

+      In the expression:

+        let

+          go :: _

+          go _ a = a

+        in go (0 :: Int) undefined

+      In an equation for ‘succeeds’:

+          succeeds

+            = let

+                go :: _

+                go _ a = a

+              in go (0 :: Int) undefined

+    • Relevant bindings include succeeds :: a (bound at T11192.hs:12:1)