Allow implicit discard when right-hand side is Void.

This is the most intuitive/expected behavior. Otherwise, it forces a pointless let-binding to 'Void' or into a discard.

examples/acceptance_tests/098/aiken.lock

@@ -0,0 +1,7 @@
+# This file was generated by Aiken
+# You typically do not need to edit this file
+
+requirements = []
+packages = []
+
+[etags]

examples/acceptance_tests/098/aiken.toml

@@ -0,0 +1,3 @@
+name = "aiken-lang/acceptance_test_098"
+version = "0.0.0"
+description = ""

examples/acceptance_tests/098/lib/foo.ak

@@ -0,0 +1,8 @@
+fn label(str: String) -> Void {
+  trace str Void
+}
+
+test foo() {
+  label(@"Foo")
+  True
+}