mre: chrome may not gc filereader output used by wasm inside webworkers

select a file which exceeds 24 MiB in size: 

hashing speed: --- MiB/s

mre variant 3, sha512-wasm:
* this version of the mre replaces `hash-wasm` with `sha512-wasm` --
  runs much slower, speed reduction from 1194 to 702 MiB/s using 4 workers,

* more importantly it reproduces the bug on a different set of chrome versions;
  depending on chrome version, gc may occur, however at uneven intervals
  and at some point not in time to avoid oom

old hasher: https://github.com/Daninet/hash-wasm (v4.9.0)
new hasher: https://github.com/chm-diederichs/sha512-wasm (v2.3.4)
sha512-wasm build steps:
* browserify -r . -s Sha512 > sha512wasm.js

Reliably observed on:
* win10-1809 (17763.3046) + chr  84.0.4115.0 (#758826, gc@appspot)   # almost instant
* win10-1809 (17763.3046) + chr 105.0.5164.0 (#1021082, gc@appspot)  # a few minutes
* win10-1809 (17763.3046) + chr 106.0.5229.0 (#1032995, gc@appspot)  # a few minutes
* win10-1809 (17763.3046) + chr 106.0.5247.0 (#1036146, gc@appspot)  # about 10 minutes
* win10-1809 (17763.316)  + chr 105.0.5195.37 (latest official gchrome beta)  # 1 min

NOT observed on:
* win10-1809 (17763.3046) + chr  88.0.4322.0 (#825998, gc@appspot)
* win10-1809 (17763.3046) + chr  90.0.4416.0 (#853132, gc@appspot)
* win10-1809 (17763.3046) + chr  93.0.4531.0 (#888620, gc@appspot)
* win10-1809 (17763.3046) + chr  93.0.4552.0 (#895174, gc@appspot)
* win10-1809 (17763.3046) + chr  95.0.4631.0 (#917620, gc@appspot)
* win10-1809 (17763.3046) + chr  97.0.4666.0 (#929984, gc@appspot)
* win10-1809 (17763.3046) + chr  97.0.4689.0 (#937004, gc@appspot)
* win10-1809 (17763.3046) + chr  98.0.4719.0 (#943825, gc@appspot)
* win10-1809 (17763.3046) + chr  98.0.4732.0 (#945698, gc@appspot)
* win10-1809 (17763.3046) + chr 102.0.4991.0 (#990124, gc@appspot)
* win10-1809 (17763.316)  + chr 104.0.5112.102 (latest official gchrome stable)