go: a counter by compare-and-set, so a number is handed out once (hq research 042)

The coming issues module hands out issue numbers from its own state; two instances must never hand out one
number twice. Create and Update already write by compare-and-set (ADR 0259), so State(...).Increment reads,
writes one more at the revision read, and on a lost write reads and tries again, bounded by the caller's
attempts. A lost write past the bound is ErrChanged, any other refusal and a value that is no counter are
returned at once: nothing is reported counted that was not written.

Part of the issue tracker decision, hq research 042.

README.md

@@ -104,6 +104,9 @@ publishes it under the module's name; `Cancel`; `Where` reads its state from the
 `Take` hand the module each warrant once — marked acted on by compare-and-set before the module acts, and only
 for an ask it made, an option it offered, at that option's level, before it expired. A lost compare-and-set is
 `stdio.ErrChanged`, told by the runtime's error code.
+`State(...).Increment(key, attempts)` is a counter on top of the two (for numbers handed out once, such as an
+issue's): it reads the key, writes one more at the revision read, and on a lost write reads and tries again, at
+most `attempts` times; past that it is `stdio.ErrChanged` and nothing was counted.
 
 ### `primitives` — sealing, semver, resolved-env
 Seal/unseal for secrets, semantic version comparison, and the accessor a module uses to read its