Cache hash code MSBs in Map buckets

[puzpuzpuz]

Closes #2

• Removes linked lists from Map buckets
  • Also increases bucket size to fit 7 entries (2 CLs). So, when compared with the current Map maximum number of entries in bucket decrease from 9 to 7 entries. On the other hand, spacial locality and data density improved
• Adds a cache with top bytes of entry hash codes
  • Improves key lookup efficiency by caching one most significant byte of each map entry. This way string dereference and equality check is avoided for a significant portion of scanned entries
• Decreases the shrink threshold fraction proportionally to the new map layout

Benchmark results

Here is the result of comparison with current main (v1.0.2) on my laptop (Ubuntu 20.04, i5-8300h, Go 1.17)

$ benchstat old.txt new.txt
name                      old time/op  new time/op  delta
Map_NoWarmUp/99%-reads-8  45.5ns ± 2%  33.5ns ± 0%  -26.31%  (p=0.008 n=5+5)
Map_NoWarmUp/90%-reads-8  59.0ns ± 3%  46.7ns ± 1%  -20.90%  (p=0.008 n=5+5)
Map_NoWarmUp/75%-reads-8  71.4ns ±23%  56.1ns ± 1%  -21.41%  (p=0.016 n=5+4)
Map_NoWarmUp/50%-reads-8  77.3ns ± 4%  77.2ns ±25%     ~     (p=0.690 n=5+5)
Map_NoWarmUp/0%-reads-8    102ns ±34%    94ns ±10%     ~     (p=1.000 n=5+5)
Map_WarmUp/100%-reads-8   64.4ns ± 0%  56.4ns ± 0%  -12.41%  (p=0.008 n=5+5)
Map_WarmUp/99%-reads-8    64.0ns ± 0%  55.8ns ± 0%  -12.77%  (p=0.008 n=5+5)
Map_WarmUp/90%-reads-8    64.7ns ± 0%  53.2ns ± 0%  -17.83%  (p=0.008 n=5+5)
Map_WarmUp/75%-reads-8    64.5ns ± 0%  53.0ns ± 0%  -17.92%  (p=0.029 n=4+4)
Map_WarmUp/50%-reads-8    68.2ns ± 9%  56.8ns ± 0%  -16.76%  (p=0.016 n=5+4)
Map_WarmUp/0%-reads-8     80.4ns ± 1%  69.3ns ± 1%  -13.79%  (p=0.016 n=5+4)

[codecov-commenter]

Codecov Report

Merging #31 (d26bcfb) into main (1fd9408) will decrease coverage by 1.98%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
- Coverage   96.66%   94.67%   -1.99%     
==========================================
  Files           5        5              
  Lines         509      488      -21     
==========================================
- Hits          492      462      -30     
- Misses         14       21       +7     
- Partials        3        5       +2     

Impacted Files	Coverage Δ
map.go	93.00% <94.73%> (-2.88%)	⬇️

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