interfaces: add an experimental toTypeRep builtin.

This adds an experimental `toTypeRep: forall t. t -> TypeRep` builtin.
It will only work on interface payloads and crash horribly otherwise.

CHANGELOG_BEGIN
CHANGELOG_END

compiler/damlc/daml-stdlib-src/DA/Experimental/Interface.daml

@@ -5,6 +5,7 @@ module DA.Experimental.Interface(
   interfaceCreate,
   interfaceSignatory,
   interfaceObserver,
+  toTypeRep,
 ) where
 
 import GHC.Types (primitive)
@@ -17,3 +18,6 @@ interfaceSignatory payload = primitive @"$RESOLVE_VIRTUAL_SIGNATORY" payload pay
 
 interfaceObserver: t -> [Party]
 interfaceObserver payload = primitive @"$RESOLVE_VIRTUAL_OBSERVER"  payload payload
+
+toTypeRep: t -> TypeRep
+toTypeRep arg = primitive @"$TO_TYPE_REP" arg

compiler/damlc/tests/daml-test-files/InterfaceExperimental.daml

@@ -8,6 +8,7 @@
 module InterfaceExperimental where
 
 import DA.Assert ((===))
+import DA.Action (unless)
 import DA.Experimental.Interface
 import GHC.Types (primitive)
 
@@ -62,13 +63,22 @@ template Asset
         do
           pure ()
 
+template NotAsset
+  with
+    p : Party
+  where
+    signatory p
+
 main = scenario do
     alice <- getParty "Alice"
     bob <- getParty "Bob"
     let asset = Asset alice bob 15
+    let notAsset = NotAsset alice
     let token = toToken asset
     submit alice do
       interfaceCreate token
       interfaceSignatory token === [alice]
       interfaceObserver token === [bob, alice]
+      unless (toTypeRep token == toTypeRep asset) $ abort "TypeReps are not equal"
+      unless (toTypeRep token /= toTypeRep notAsset) $ abort "TypeReps are equal"
       pure ()

daml-lf/interpreter/src/main/scala/com/digitalasset/daml/lf/speedy/SBuiltin.scala

@@ -1667,9 +1667,17 @@ private[lf] object SBuiltin {
         machine.returnValue = SInt64(42L)
     }
 
+    private object SBExperimentalToTypeRep extends SBuiltinPure(1) {
+      override private[speedy] def executePure(args: util.ArrayList[SValue]): STypeRep = {
+        val id = getSRecord(args, 0).id
+        STypeRep(Ast.TTyCon(id))
+      }
+    }
+
     private val mapping: Map[String, SExpr] =
       List(
         "ANSWER" -> SBExperimentalAnswer,
+        "TO_TYPE_REP" -> SBExperimentalToTypeRep,
         "RESOLVE_VIRTUAL_CREATE" -> new SBResolveVirtual(CreateDefRef),
         "RESOLVE_VIRTUAL_SIGNATORY" -> new SBResolveVirtual(SignatoriesDefRef),
         "RESOLVE_VIRTUAL_OBSERVER" -> new SBResolveVirtual(ObserversDefRef),