Order inserts before deletes

Two deltas can be considered equal if they both produce the same
resulting documents, for all initial documents. We would like the
property that equality can be detected by a simple deep comparison. This
was not previously true since the following deltas were functionally
equal but did not deep compare:

var a = new Delta().insert('a').delete(1);
var b = new Delta().delete(1).insert('a');

Without this property fuzzer with seed 65353 fails since it uses deep
comparison.