💥 Only support node>=8 and ES2017+

[dubzzz]

Merge of PR #515 to master

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Why is this PR for?

Remove support for old versions of Node.

Removing the support brings a clear uplift in terms of performances, here are the results we measured on RunKit by running the same code against fast-check@1.26.0 and fast-check@2.0.0-alpha.1.26.0.0:

• synchronous property: from 0.30ms to 0.26ms (-14% on median time)
• asynchronous property: from 0.54ms to 0.36ms (-34% on median time)

// synchronous property
(fc, seed) => {
  const start = performance.now();
  fc.assert(fc.property(fc.nat(), fc.nat(), (a, b) => true), {seed});
  return performance.now() - start;
}
// asynchrounous property
async (fc, seed) => {
  const start = performance.now();
  await fc.assert(fc.asyncProperty(fc.nat(), fc.nat(), async (a, b) => true), {seed});
  return performance.now() - start;
}

Most of the uplift might be related to the fact that neither yield nor async/await need to be transpiled if we drop support for older releases on nodes.

In a nutshell

❌ New feature
❌ Fix an issue
❌ Documentation improvement
✔️ Other: remove support

(✔️: yes, ❌: no)

Potential impacts

Code will no longer run on old versions of node.

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[coveralls]

Coverage decreased (-0.2%) to 95.579% when pulling cc846ab on support-node-8 into 84aa5a7 on master.