Addressing comments in Flurry

[JackThomson2]

This is to address comments made in this PR: jonhoo/flurry#109

Key points:

• Removed the option for nightly
• No longer using the unsafe cell.
• Aligning the cells the the size of a cach-line to help with cache invalidation

Some notes: I did look at using the thread_id but this is a nightly only API so I'll leave it out for now as we're fully working on stable

I'm seeing some suspiciously good benchmark results:

atomic_counter/2        time:   [266.96 us 271.03 us 274.81 us]
                        thrpt:  [119.24 Melem/s 120.90 Melem/s 122.74 Melem/s]

atomic_counter/4        time:   [337.74 us 339.58 us 341.31 us]
                        thrpt:  [96.008 Melem/s 96.497 Melem/s 97.021 Melem/s]

atomic_counter/8        time:   [355.62 us 358.51 us 361.44 us]
                        thrpt:  [90.660 Melem/s 91.400 Melem/s 92.142 Melem/s]

atomic_counter/16       time:   [447.33 us 450.47 us 453.74 us]
                        thrpt:  [72.217 Melem/s 72.742 Melem/s 73.252 Melem/s]

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

fast_counter_stable/2   time:   [26.046 us 26.210 us 26.363 us]
                        thrpt:  [1.2430 Gelem/s 1.2502 Gelem/s 1.2581 Gelem/s]

fast_counter_stable/4   time:   [16.023 us 16.117 us 16.219 us]
                        thrpt:  [2.0204 Gelem/s 2.0331 Gelem/s 2.0451 Gelem/s]

fast_counter_stable/8   time:   [24.720 us 25.833 us 26.912 us]
                        thrpt:  [1.2176 Gelem/s 1.2685 Gelem/s 1.3256 Gelem/s]

fast_counter_stable/16  time:   [50.007 us 50.833 us 51.691 us]
                        thrpt:  [633.92 Melem/s 644.62 Melem/s 655.27 Melem/s]

However it looked like we were actually being bottlenecked by the allocation time, when I increase the iter count in the benchmark we can see great scaling: at 16 core we're seeing a near 60x improvment over regular atomic numbers

atomic_counter/1        time:   [1.5215 ms 1.5311 ms 1.5414 ms]
                        thrpt:  [680.28 Melem/s 684.86 Melem/s 689.19 Melem/s]

atomic_counter/2        time:   [8.1665 ms 8.3089 ms 8.4518 ms]
                        thrpt:  [124.07 Melem/s 126.20 Melem/s 128.40 Melem/s]

atomic_counter/4        time:   [10.192 ms 10.250 ms 10.303 ms]
                        thrpt:  [101.77 Melem/s 102.30 Melem/s 102.88 Melem/s]

atomic_counter/8        time:   [10.021 ms 10.131 ms 10.234 ms]
                        thrpt:  [102.46 Melem/s 103.50 Melem/s 104.64 Melem/s]

atomic_counter/16       time:   [12.362 ms 12.393 ms 12.425 ms]
                        thrpt:  [84.390 Melem/s 84.610 Melem/s 84.823 Melem/s]

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

fast_counter_stable/1   time:   [1.5678 ms 1.5795 ms 1.5913 ms]
                        thrpt:  [658.92 Melem/s 663.88 Melem/s 668.81 Melem/s]

fast_counter_stable/2   time:   [820.95 us 824.81 us 828.56 us]
                        thrpt:  [1.2655 Gelem/s 1.2713 Gelem/s 1.2773 Gelem/s]

fast_counter_stable/4   time:   [429.10 us 430.62 us 432.17 us]
                        thrpt:  [2.4263 Gelem/s 2.4350 Gelem/s 2.4437 Gelem/s]

fast_counter_stable/8   time:   [240.59 us 242.92 us 245.44 us]
                        thrpt:  [4.2723 Gelem/s 4.3165 Gelem/s 4.3583 Gelem/s]

fast_counter_stable/16  time:   [206.01 us 210.24 us 214.74 us]
                        thrpt:  [4.8829 Gelem/s 4.9875 Gelem/s 5.0900 Gelem/s]