[WIP] implement slice-by-4 with SIMD instructions, take 2

[Touffy]

This is an alternate version of client-zip with SIMD-enabled CRC32, where the basic algorithm is loaded synchronously as before, and the optimised version can be swapped in later from an external WASM file.

pros :

• downloadZip is still immediately usable after importing
• failing to load the new WASM file for any reason will not break anything
• the new WASM can be safely swapped in while downloadZip is running !
• no base64 encoding for the larger WASM (avoids the large, ugly string in the source code)

cons :

• users may forget to call useSimd, or to host the WASM file in the right place
• useSimd itself takes up a few hundred bytes (though less than the whole WASM)