fix: segfaults when used with worker_threads

[mariuspod]

This PR makes the lib more robust when used in combination with worker_threads.
We've observed that the usage of the global constructor field is not thread-safe and may lead to segfaults.

Fixes issue: #172

Minimal segfaulting example without this change:

const wt = require('worker_threads');
const foo = require('secp256k1');

if(wt.isMainThread) {
  for(let i = 0; i < 100; i++) {
    new wt.Worker(__filename);
  }
}

[fanatid]

@mariuspod can you update it as keccak? cryptocoinjs/keccak#24

[fanatid]

@mariuspod thank you! I finished PR in #195 and published as v5.0.0