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.

lib/cretonne/src/ir/stackslot.rs

@@ -8,7 +8,7 @@ use ir::{Type, StackSlot};
 use packed_option::PackedOption;
 use std::cmp;
 use std::fmt;
-use std::ops::Index;
+use std::ops::{Index, IndexMut};
 use std::str::FromStr;
 
 /// The size of an object on the stack, or the size of a stack frame.
@@ -228,6 +228,12 @@ impl Index<StackSlot> for StackSlots {
     }
 }
 
+impl IndexMut<StackSlot> for StackSlots {
+    fn index_mut(&mut self, ss: StackSlot) -> &mut StackSlotData {
+        &mut self.slots[ss]
+    }
+}
+
 /// Higher-level stack frame manipulation functions.
 impl StackSlots {
     /// Create a new spill slot for spilling values of type `ty`.