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, 17 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/ExPat.hs b/testsuite/tests/typecheck/should_compile/ExPat.hs
new file mode 100644
index 0000000000..a0b4b0b007
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/ExPat.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE ViewPatterns, GADTs #-}
+
+module ExPat where
+
+data T where
+  MkT :: Integral a => a -> Int -> T
+
+-- c.f. T11700
+
+-- Succeeds becuase y::Int
+f x = let MkT _ y  = x
+      in y
+
+-- Remarkablly, this succeeds because
+--   (toInteger (v::a)) is an Integer
+g x = let MkT (toInteger -> y) _ = x
+      in y