Polymorphism fixes (#415)

* Assume that void is a supertype of all types

* Add test for void supertype

* Unify functions with decls from implemented interfaces

* Rename delete_if_implementation

* Match only with function name and without typesig

test/contracts/polymorphism_variance_switching_unify_with_interface_decls.aes

@@ -0,0 +1,5 @@
+payable contract interface SalesOffer =
+    entrypoint init : (address, address, int, int) => unit
+
+payable contract Test : SalesOffer =
+    entrypoint init(_, _, _, _) = ()

test/contracts/polymorphism_variance_switching_void_supertype.aes

@@ -0,0 +1,5 @@
+payable contract interface SalesOffer =
+    entrypoint init : (address, address, int, int) => void
+
+payable contract Test : SalesOffer =
+    entrypoint init(_ : address, _ : address, _ : int, _ : int) = ()