Use bytes8 instead of bytes32 as hash key

[cgewecke]

This should reduce the instrumentation text-size footprint a quite a bit.

[codecov-io]

Codecov Report

Merging #590 (9d2cade) into experimental-options (235866f) will not change coverage.
The diff coverage is 100.00%.

@@                  Coverage Diff                  @@
##           experimental-options     #590   +/-   ##
=====================================================
  Coverage                 99.09%   99.09%           
=====================================================
  Files                        22       22           
  Lines                      1109     1109           
=====================================================
  Hits                       1099     1099           
  Misses                       10       10           

Impacted Files	Coverage Δ
lib/collector.js	100.00% <100.00%> (ø)
lib/coverage.js	100.00% <100.00%> (ø)
lib/injector.js	100.00% <100.00%> (ø)

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