Compute enum discriminant during parsing

This allows us to reuse the computation in multiple places later.
diff --git a/syntax/mod.rs b/syntax/mod.rs
index e6c5bb8..fd8db73 100644
--- a/syntax/mod.rs
+++ b/syntax/mod.rs
@@ -95,7 +95,7 @@
 
 pub struct Variant {
     pub ident: Ident,
-    pub discriminant: Option<u32>,
+    pub discriminant: u32,
 }
 
 pub enum Type {