Don't renumber entities in the parser.

This makes it easier to debug testcases:
 - the entity numbers in a .cton file match the entity numbers used
   within Cretonne.
 - serializing and deserializing doesn't cause indices to change.

One disadvantage is that if a .cton file uses sparse entity numbers,
deserializing to the in-memory form doesn't compact it. However, the
text format is not intended to be performance-critical, so this isn't
expected to be a big burden.

cranelift/filetests/isa/intel/binary32.cton

@@ -9,8 +9,8 @@ isa intel haswell
 ;
 
 function %I32() {
-    fn0 = function %foo()
     sig0 = ()
+    fn0 = function %foo()
 
     gv0 = globalsym %some_gv