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/parser/rewrite.cton

@@ -30,8 +30,8 @@ ebb100(v20: i32):
     jump ebb100(v1000)
 }
 ; sameln: function %use_value() native {
-; nextln: ebb0($v20: i32):
+; nextln: ebb100($v20: i32):
 ; nextln:     $v1000 = iadd_imm $v20, 5
 ; nextln:     $v200 = iadd $v20, $v1000
-; nextln:     jump ebb0($v1000)
+; nextln:     jump ebb100($v1000)
 ; nextln: }