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/riscv/binary32.cton

@@ -3,8 +3,8 @@ test binemit
 isa riscv
 
 function %RV32I(i32 link [%x1]) -> i32 link [%x1] {
-    fn0 = function %foo()
     sig0 = ()
+    fn0 = function %foo()
 
 ebb0(v9999: i32):
     [-,%x10]            v1 = iconst.i32 1