Faster uint decoding

[michaelsproul]

This PR optimises integer decoding by inlining the decoding functions aggressively.

The speedup is around 60% for u8 per benchmarks from this PR:

• Optimise SSZ encoding and decoding ssz_types#55

decode_fixed_vector/u8_128k
                        time:   [118.56 µs 119.22 µs 119.99 µs]
                        change: [−61.759% −61.667% −61.573%] (p = 0.00 < 0.05)
                        Performance has improved.

Inlining is the main source of gains, the "simpler" FastU8 impl from the ssz_types PR actually turns out to be 3% slower than just inlining the current impl.

TODO:

• Confirm performance benefit for other uint types.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.16%. Comparing base (5b6dd17) to head (2afb83d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
+ Coverage   75.59%   76.16%   +0.56%     
==========================================
  Files          13       13              
  Lines         795      797       +2     
==========================================
+ Hits          601      607       +6     
+ Misses        194      190       -4     

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[jimmygchen]

Great find 🎉

[michaelsproul]

The benchmarks for u64 lists are also greatly improved by these changes:

fixed_vector/decode_u64_128k
                        time:   [72.013 µs 72.038 µs 72.066 µs]
                        change: [−63.833% −63.646% −63.454%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 11 outliers among 100 measurements (11.00%)
  4 (4.00%) high mild
  7 (7.00%) high severe

fixed_vector/encode_u64_128k
                        time:   [67.006 µs 67.145 µs 67.299 µs]
                        change: [−70.382% −70.308% −70.223%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  8 (8.00%) high mild
  1 (1.00%) high severe

variable_list/decode_u64_128k
                        time:   [71.978 µs 72.135 µs 72.257 µs]
                        change: [−57.648% −57.407% −57.177%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 12 outliers among 100 measurements (12.00%)
  6 (6.00%) low severe
  5 (5.00%) low mild
  1 (1.00%) high mild

variable_list/encode_u64_128k
                        time:   [82.348 µs 82.562 µs 82.832 µs]
                        change: [−56.293% −55.142% −53.937%] (p = 0.00 < 0.05)
                        Performance has improved.