Simpler syncer specs

This simplifies the syncer specs. The sequential syncer only is slightly improved, by changing

  Definition synced R (f' f: val) :=
    (□ ∀ P Q (x: val), ({{ R ★ P x }} f x {{ v, R ★ Q x v }}) →
                       ({{ P x }} f' x {{ v, Q x v }}))%I.

such that P, Q don't factor in a dependency on x any more. Since all three are quantified at the same time, the two specs are equivalent -- and the one where P is not a predicate and Q is exactly the postcondition is arguably simpler:

  Definition synced R (f f': val) :=
    (□ ∀ P Q (x: val), ({{ R ★ P }} f x {{ v, R ★ Q v }}) →
                       ({{ P }} f' x {{ Q }}))%I.

For the atomic spec, the changes are deeper. The currying on some location l is entirely removed. is_atomic_syncer better mirrors is_syncer from the sequential version. And finally, we use "original" atomic triples and not some modified version thereof (so atomic_triple_base is killed).

TODO: Fix atomic_pcas.v, it relied on the location currying done by atomic_sync.v. However, I don't entirely grok that file. @zhangz could you give that a look?