perf: improve performance of emit()

[Uzlopak]

Follow Up of #69

As @climba03003 correctly concluded, checking first if we already emitted the warning is actually the performance gain. So we put that to the fast-path.

before:

uzlopak@Battlestation:~/workspace/fastify-org/process-warning$ node benchmarks/warn.js
warn x 40,120,473 ops/sec ±0.64% (98 runs sampled)
(node:9821) [FST_ERROR_CODE_1] FastifyWarning: message
(Use `node --trace-warnings ...` to show where the warning was created)
(node:9821) [FST_ERROR_CODE_3] FastifyWarning: message

after:

uzlopak@Battlestation:~/workspace/fastify-org/process-warning$ node benchmarks/warn.js
warn x 90,812,134 ops/sec ±0.27% (94 runs sampled)
(node:9882) [FST_ERROR_CODE_1] FastifyWarning: message
(Use `node --trace-warnings ...` to show where the warning was created)
(node:9882) [FST_ERROR_CODE_3] FastifyWarning: message

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm