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/mod.rs b/syntax/mod.rs
index c5cc1de..1acb9a5 100644
--- a/syntax/mod.rs
+++ b/syntax/mod.rs
@@ -9,9 +9,11 @@
 mod impls;
 mod parse;
 pub mod set;
+mod span;
 mod tokens;
 pub mod types;
 
+use self::span::Span;
 use proc_macro2::Ident;
 use syn::{LitStr, Token};
 
@@ -70,6 +72,7 @@
     UniquePtr(Box<Ty1>),
     Ref(Box<Ref>),
     Str(Box<Ref>),
+    Void(Span),
 }
 
 pub struct Ty1 {