std.hash_map: fetchRemove increment available

To avoid leaking slots, `fetchRemove` must increment `available` (when the "fetch" succeeds).

Without the `available += 1`, the added test `"std.hash_map repeat fetchRemove"` fails with

    run test std-x86-linux-none-Debug: error: thread 432734 panic: integer overflow
    .../zig/lib/std/hash_map.zig:1365:28: 0x6471d5 in getOrPutAssumeCapacityAdapted__anon_47495 (test)
            self.available -= 1;
                           ^
    .../zig/lib/std/hash_map.zig:1308:62: 0x616950 in getOrPutAssumeCapacityContext (test)
            const result = self.getOrPutAssumeCapacityAdapted(key, ctx);
                                                             ^

Alternatively, `fetchRemove` could call `removeByIndex`, though that would entail calling `header()` twice instead of once.

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