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/typecheck/should_fail/mc19.stderr b/testsuite/tests/typecheck/should_fail/mc19.stderr
index 5f004dc4a8..4b00eeaecc 100644
--- a/testsuite/tests/typecheck/should_fail/mc19.stderr
+++ b/testsuite/tests/typecheck/should_fail/mc19.stderr
@@ -1,12 +1,12 @@
-
-mc19.hs:10:31: error:
-    • Couldn't match type ‘a’ with ‘[a]’
-      ‘a’ is a rigid type variable bound by
-        a type expected by the context:
-          forall a. [a] -> [a]
-        at mc19.hs:10:10
-      Expected type: [a] -> [a]
-        Actual type: [a] -> [[a]]
-    • In the expression: inits
-      In a stmt of a monad comprehension: then inits
-      In the expression: [x | x <- [3, 2, 1], then inits]
+

+mc19.hs:10:31: error:

+    • Couldn't match type ‘a’ with ‘[a]’

+      ‘a’ is a rigid type variable bound by

+        a type expected by the context:

+          forall a. [a] -> [a]

+        at mc19.hs:10:10-35

+      Expected type: [a] -> [a]

+        Actual type: [a] -> [[a]]

+    • In the expression: inits

+      In a stmt of a monad comprehension: then inits

+      In the expression: [x | x <- [3, 2, 1], then inits]