Use a single Arc shared between Wakers in WakerArray and WakerVec

[wishawa]

This PR is dependent on #117

after = this PR before = #117

group                after                                  before
-----                -----                                  ------
array::join 10       1.00      2.3±0.12µs        ? ?/sec    1.13      2.6±0.20µs        ? ?/sec
array::join 100      1.00     23.5±0.71µs        ? ?/sec    1.17     27.5±0.39µs        ? ?/sec
array::join 1000     1.10  1521.6±41.26µs        ? ?/sec    1.00  1377.7±21.95µs        ? ?/sec
array::merge 10      1.00      2.3±0.04µs        ? ?/sec    1.39      3.3±0.07µs        ? ?/sec
array::merge 100     1.00     52.5±6.18µs        ? ?/sec    1.09     57.5±2.20µs        ? ?/sec
array::merge 1000    1.00      2.5±0.06ms        ? ?/sec    1.04      2.6±0.07ms        ? ?/sec
array::race 10       1.00  1158.9±19.21ns        ? ?/sec    1.00  1159.8±25.91ns        ? ?/sec
array::race 100      1.00     12.2±0.33µs        ? ?/sec    1.02     12.5±0.48µs        ? ?/sec
array::race 1000     1.02    144.0±5.25µs        ? ?/sec    1.00    140.7±1.84µs        ? ?/sec
tuple::join 10       1.00      2.1±0.04µs        ? ?/sec    1.17      2.5±0.06µs        ? ?/sec
tuple::merge 10      1.00      2.8±0.06µs        ? ?/sec    1.10      3.1±0.07µs        ? ?/sec
tuple::race 10       1.05  1147.9±36.64ns        ? ?/sec    1.00  1089.3±19.36ns        ? ?/sec
vec::join 10         1.00      2.5±0.03µs        ? ?/sec    1.16      2.9±0.09µs        ? ?/sec
vec::join 100        1.00     34.5±1.06µs        ? ?/sec    1.15     39.6±0.80µs        ? ?/sec
vec::join 1000       1.07  1989.7±40.57µs        ? ?/sec    1.00  1863.9±26.66µs        ? ?/sec
vec::merge 10        1.00      2.9±0.13µs        ? ?/sec    1.25      3.6±0.06µs        ? ?/sec
vec::merge 100       1.00     57.3±1.04µs        ? ?/sec    1.28     73.3±1.38µs        ? ?/sec
vec::merge 1000      1.00      3.6±0.07ms        ? ?/sec    1.17      4.2±0.08ms        ? ?/sec
vec::race 10         1.01  1157.2±18.82ns        ? ?/sec    1.00  1147.6±14.78ns        ? ?/sec
vec::race 100        1.03     12.0±0.19µs        ? ?/sec    1.00     11.6±0.23µs        ? ?/sec
vec::race 1000       1.02    141.4±5.12µs        ? ?/sec    1.00    138.5±3.85µs        ? ?/sec

Also: made dummy waker easier to use and allocation-free.

[wishawa]

@yoshuawuyts thanks for the review! I added more documentation/comments, renamed things so that they (hopefully) make more sense, and simplified the code quite a bit[1]. Please take a look!

[1] The simplification fixed the

if BY_REF {
            std::mem::forget(arc);
}

and

            wake::<A, false>,
            wake::<A, true>,

weirdness.

Previously I was using Arc::from_raw to get access to the Arc content, and dropping/forgetting the created Arc depending on whether the reference count should be decremented. In the updated code I dereference the raw pointer directly and, as you suggested, use Arc::decrement_strong_count properly.

[wishawa]

@yoshuawuyts @matheus-consoli is there still interest in this change? I think reducing the number of allocations from N to 1 is very beneficial.