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.

docs/langref.rst

@@ -394,11 +394,11 @@ preamble`:
 This simple example illustrates direct function calls and signatures::
 
     function gcd(i32 uext, i32 uext) -> i32 uext "C" {
-        f1 = function divmod(i32 uext, i32 uext) -> i32 uext, i32 uext
+        fn1 = function divmod(i32 uext, i32 uext) -> i32 uext, i32 uext
 
     ebb1(v1: i32, v2: i32):
         brz v2, ebb2
-        v3, v4 = call f1(v1, v2)
+        v3, v4 = call fn1(v1, v2)
         br ebb1(v2, v4)
 
     ebb2: