Remove Slice and upgrade len and is_empty to const-fns

[bhgomes]

The len and is_empty functions of the four PHF types are upgraded to const-fns.

Implementation Detail

In this implementation, I could only upgrade them when the std feature is turned off because of the definition of Slice. I'm not sure what the purpose of the Dynamic variant is on Slice. Is there any reason we can't switch to slice?

[JohnTitor]

In this implementation, I could only upgrade them when the std feature is turned off because of the definition of Slice. I'm not sure what the purpose of the Dynamic variant is on Slice. Is there any reason we can't switch to slice?

The Dynamic variant was used to be required by the phf_builder crate, which has been removed now (ref. 67b00ba). So, there's no use of it now and I think we can safely remove the enum.

[JohnTitor]

LGTM, thanks!