Add FuncRef and SigRef entity references.

These refer to external functions and function signatures declared in
the preamble. Since we're already using the type names 'Signature' and
'Function', these entity references don't folow the usual EntityData /
Entity naming convention.

src/libreader/lexer.rs

@@ -34,6 +34,8 @@ pub enum Token<'a> {
     Ebb(Ebb), // ebb3
     StackSlot(u32), // ss3
     JumpTable(u32), // jt2
+    FuncRef(u32), // fn2
+    SigRef(u32), // sig2
     Identifier(&'a str), // Unrecognized identifier (opcode, enumerator, ...)
 }
 
@@ -285,6 +287,8 @@ impl<'a> Lexer<'a> {
             "ebb" => Ebb::with_number(number).map(|ebb| Token::Ebb(ebb)),
             "ss" => Some(Token::StackSlot(number)),
             "jt" => Some(Token::JumpTable(number)),
+            "fn" => Some(Token::FuncRef(number)),
+            "sig" => Some(Token::SigRef(number)),
             _ => None,
         }
     }