Fix: Get the type of Chain.create() from its application (#407)

* Get the type of Chain.create() from its application

* Add test contract

* Update CHANGELOG.md

* Update the tests

* Update tests again

test/contracts/polymorphism_variance_switching_chain_create.aes

@@ -0,0 +1,29 @@
+contract interface I =
+    entrypoint f : () => int
+
+contract C1 : I =
+    entrypoint f() = 123
+
+contract C2 : I =
+    entrypoint f() = 888
+
+namespace Make =
+    stateful function new1() : I = Chain.create() : C1
+    stateful function new2() : I = Chain.create() : C2
+
+    stateful function new(c : I) : int = c.f()
+
+main contract Main =
+    stateful entrypoint test1() =
+        let c = Make.new1()
+        Make.new(c)
+
+    stateful entrypoint test2() =
+        let c = Make.new2()
+        Make.new(c)
+
+    stateful entrypoint test3() =
+        let c1      = Chain.create() : C1  // succeeds
+        let c2 : I  = Chain.create() : C1  // succeeds
+        let c3 : C1 = Chain.create()       // succeeds
+        ()

test/contracts/polymorphism_variance_switching_chain_create_fail.aes

@@ -0,0 +1,12 @@
+contract interface I =
+    entrypoint f : () => int
+
+contract C : I =
+    entrypoint f() = 123
+
+main contract Main =
+    stateful entrypoint test() =
+        let c1 : I = Chain.create()       // fails
+        let c2 : C = Chain.create() : I   // fails
+        let c3     = Chain.create() : I   // fails
+        ()