Node.js v24: `SlowBuffer` runtime deprecation

[silverwind]

Since Node.js v24, SlowBuffer issues runtime deprecation warnings whenever this library is used:

https://nodejs.org/api/deprecations.html#DEP0030

The fix in #456 should be pushed to the v5x branch and be released in a patch release.

[silverwind]

#463 would fix this.

[mtth]

Thanks @silverwind. This has been fixed in v5.7.8.