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/reader/src/testfile.rs

@@ -36,8 +36,7 @@ pub struct Details<'a> {
     pub location: Location,
     /// Annotation comments that appeared inside or after the function.
     pub comments: Vec<Comment<'a>>,
-    /// Mapping of source entity numbers to parsed entity numbers.
-    /// Source locations of parsed entities.
+    /// Mapping of entity numbers to source locations.
     pub map: SourceMap,
 }