Add Type::Void variant

Not currently usable as a function argument or explicit return value,
but will be required when we introduce Result for the case of fallible
void functions, whose return type will be Result<()>.
diff --git a/syntax/check.rs b/syntax/check.rs
index df633de..81e1f05 100644
--- a/syntax/check.rs
+++ b/syntax/check.rs
@@ -161,6 +161,7 @@
         Type::UniquePtr(_) => "unique_ptr".to_owned(),
         Type::Ref(_) => "reference".to_owned(),
         Type::Str(_) => "&str".to_owned(),
+        Type::Void(_) => "()".to_owned(),
     }
 }