v8.0.0

Breaking

• Convert the package to TypeScript (#64) 0b5c364
  The types might be slightly different than v6.
• Require Node.js 10 (#67) 1afa53c
• Throw on .clear(fn) if fn’s cache can't be cleared (#59) e7c8893

v6.1.1...v8.0.0

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What happened to v7? This version was prematurely published as v7, so we did v8 to prevent breaking for those already on v7.