Replace use of reflect.SliceHeader with unsafe.Slice

[shueybubbles]

The string encoding optimizations rely on reflect.SliceHeader and should be changed to reflect its deprecation.

[shueybubbles]

@PeteBassettBet365 is this something you could work on, as the author of the original optimization code?

[PeteBassettBet365]

Hi, I can take a look