atomic persistentMap.set()

Fixes #46 This patch stays under the existing size limits. I created two new tests to confirm the new atomic set functionality. I also had to "fix" this test: https://github.com/nanostores/persistent/blob/1a0b0e0d6f09047d6bb826d53a69144cb1c01dc9/test/map.test.ts#L38-L58 because its event list includes the repeated events (i.e. firing two individual events for both keys being set in one `set()` call). I am not sure whether this requires a major semver bump - someone could conceivably be relying on the (buggy) repeated notifications, that would be quite idiosyncratic but updating this would change (and could break) how their listeners get notified.