Eliminate the "past" package.

This package was always included in apps, and even if it was possible to remove,
there wasn't a compelling story about when users would remove and replace
it. Plus, not all backwards-compatibility code could even live in it (eg, field
names of objects), so it was incomplete. It also introduced odd load order
constraints.

Instead, we introduce two conventions for backwards-compatibility code:

  - Special comments of the form "// XXX COMPAT WITH 0.6.4"
  - When feasible, put backwards-compatibility code in a file called
    "deprecated.js" in the relevant package.

This is documented at:
https://github.com/meteor/meteor/wiki/Meteor-Style-Guide#deprecated-code-and-backwards-compatibility

Additionally, removed some symbols that existed for backwards compatibility with
Meteor 0.4.0 (changes made 10 months ago): Meteor.is_client, Meteor.is_server,
and (in a method) this.is_simulation.
Push through on the Meteor.connect -> DDP.connect rename.