Fix #16293 by cleaning up Proxy# infelicities

This bug fixes three problems related to `Proxy#`/`proxy#`:

1. Reifying it with TH claims that the `Proxy#` type constructor has
   two arguments, but that ought to be one for consistency with
   TH's treatment for other primitive type constructors like `(->)`.
   This was fixed by just returning the number of
   `tyConVisibleTyVars` instead of using `tyConArity` (which includes
   invisible arguments).
2. The role of `Proxy#`'s visible argument was hard-coded as nominal.
   Easily fixed by changing it to phantom.
3. The visibility of `proxy#`'s kind argument was specified, which
   is different from the `Proxy` constructor (which treats it as
   inferred). Some minor refactoring in `proxyHashId` fixed ths up.

   Along the way, I had to introduce a `mkSpecForAllTy` function, so
   I did some related Haddock cleanup in `Type`, where that function
   lives.

2 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/primops/should_compile/T16293a.hs b/testsuite/tests/primops/should_compile/T16293a.hs
new file mode 100644
index 0000000000..69368c70b5
--- /dev/null
+++ b/testsuite/tests/primops/should_compile/T16293a.hs
@@ -0,0 +1,19 @@
+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE TypeApplications #-}
+module T16293a where
+
+import Data.Coerce
+import Data.Proxy
+import GHC.Exts
+
+test1a :: () -> Proxy Int
+test1a _ = Proxy @Int
+
+test1b :: () -> Proxy# Int
+test1b _ = proxy# @Int
+
+test2a :: (() -> Proxy a) -> (() -> Proxy b)
+test2a = coerce
+
+test2b :: (() -> Proxy# a) -> (() -> Proxy# b)
+test2b = coerce
diff --git a/testsuite/tests/primops/should_compile/all.T b/testsuite/tests/primops/should_compile/all.T
index a934e4ca9e..aa7339c445 100644
--- a/testsuite/tests/primops/should_compile/all.T
+++ b/testsuite/tests/primops/should_compile/all.T
@@ -1 +1,2 @@
 test('T6135_should_compile', normal, compile, [''])
+test('T16293a', normal, compile, [''])