buffer: improve `btoa` performance

Improves the performance of the legacy btoa function. It's not recommended to be used in production.

                                            confidence improvement accuracy (*)    (**)   (***)
buffers/buffer-btoa.js n=1000000 size=1024        ***   1089.00 %      ±15.27% ±20.57% ±27.29%
buffers/buffer-btoa.js n=1000000 size=128         ***    571.99 %       ±6.85%  ±9.22% ±12.22%
buffers/buffer-btoa.js n=1000000 size=16          ***    306.04 %       ±4.24%  ±5.70%  ±7.53%
buffers/buffer-btoa.js n=1000000 size=256         ***    870.77 %      ±13.85% ±18.67% ±24.78%
buffers/buffer-btoa.js n=1000000 size=32          ***    350.58 %       ±4.61%  ±6.18%  ±8.15%
buffers/buffer-btoa.js n=1000000 size=64          ***    462.57 %       ±5.60%  ±7.51%  ±9.91%

Disclaimer

"btoa" is a legacy functionality, and not recommended to be used in production applications.

Thanks Matthieu Riegler for the poster