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/verifier/memory.cton

@@ -1,7 +1,7 @@
 test verifier
 
 function %deref_cycle() {
-    gv1 = deref(gv2)-32 ; error: deref cycle: [gv0, gv1]
+    gv1 = deref(gv2)-32 ; error: deref cycle: [gv1, gv2]
     gv2 = deref(gv1)
 
 ebb1: