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, 12 insertions, 12 deletions

diff --git a/testsuite/tests/module/mod71.stderr b/testsuite/tests/module/mod71.stderr
index d02aac28fa..f0f43d8df6 100644
--- a/testsuite/tests/module/mod71.stderr
+++ b/testsuite/tests/module/mod71.stderr
@@ -1,12 +1,12 @@
-
-mod71.hs:4:9: error:
-    • Found hole: _ :: t1
-      Where: ‘t1’ is a rigid type variable bound by
-               the inferred type of f :: Num t => (t1 -> t -> t2) -> t2
-               at mod71.hs:4:1
-    • In the first argument of ‘x’, namely ‘_’
-      In the expression: x _ 1
-      In an equation for ‘f’: f x = x _ 1
-    • Relevant bindings include
-        x :: t1 -> t -> t2 (bound at mod71.hs:4:3)
-        f :: (t1 -> t -> t2) -> t2 (bound at mod71.hs:4:1)
+

+mod71.hs:4:9: error:

+    • Found hole: _ :: t1

+      Where: ‘t1’ is a rigid type variable bound by

+               the inferred type of f :: Num t => (t1 -> t -> t2) -> t2

+               at mod71.hs:4:1-11

+    • In the first argument of ‘x’, namely ‘_’

+      In the expression: x _ 1

+      In an equation for ‘f’: f x = x _ 1

+    • Relevant bindings include

+        x :: t1 -> t -> t2 (bound at mod71.hs:4:3)

+        f :: (t1 -> t -> t2) -> t2 (bound at mod71.hs:4:1)