PFFFT benchmark

Compare the performance of the SIMD and non SIMD versions of WASM PFFFT. A pure javascript FFT library (FFT.js) severves as a baseline. The benchmark does a real forward transform of 1024 samples for 100k times. This benchmark is especially relevant for audio applications.

Note that CORS needs to be configured correctly to make this demo work. A working configuration sets these headers: 'Cross-Origin-Embedder-Policy' => 'require-corp' and 'Cross-Origin-Opener-Policy' => 'same-origin'. Additionally a browser is needed with SIMD WASM support.