P.S. @djdolphin: Never mind, the problem I noticed is that the path to the library is now .../snap/libraries rather than .../snapsource/libraries. I have a fix for that.
But a more serious problem that I don't know how to fix is that if you turn on bignums and then you load a different project into the same Snap! instance (or start a new one with ^N), bignums are still turned on even though the library blocks aren't available in the palette. That must have been true all along!