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, 9 insertions, 9 deletions

diff --git a/testsuite/tests/polykinds/T11399.stderr b/testsuite/tests/polykinds/T11399.stderr
index 5e09870088..4af1c57b75 100644
--- a/testsuite/tests/polykinds/T11399.stderr
+++ b/testsuite/tests/polykinds/T11399.stderr
@@ -1,9 +1,9 @@
-
-T11399.hs:7:32: error:
-    • Couldn't match kind ‘*’ with ‘GHC.Types.RuntimeRep’
-      When matching kinds
-        k :: * -> *
-        TYPE :: GHC.Types.RuntimeRep -> *
-      Expected kind ‘* -> *’, but ‘UhOh k’ has kind ‘k * -> *’
-    • In the first argument of ‘Functor’, namely ‘UhOh k’
-      In the instance declaration for ‘Functor (UhOh k)’
+

+T11399.hs:10:32: error:

+    • Couldn't match kind ‘*’ with ‘GHC.Types.RuntimeRep’

+      When matching kinds

+        a :: * -> *

+        TYPE :: GHC.Types.RuntimeRep -> *

+      Expected kind ‘* -> *’, but ‘UhOh a’ has kind ‘a * -> *’

+    • In the first argument of ‘Functor’, namely ‘UhOh a’

+      In the instance declaration for ‘Functor (UhOh a)’