Use the API as intended by the subtle crate authors

[sebastianv89]

This is admittedly a very nit-picky change, but according to the Subtle documentation (https://docs.rs/subtle/latest/subtle/struct.Choice.html#note), a Choice should be converted to a bool using the From implementation. Specifically they added the following note to the unwrap_u8 documentation:

This function only exists as an escape hatch for the rare case where it’s not possible to use one of the subtle-provided trait impls. To convert a Choice to a bool, use the From implementation instead.

[tarcieri]

That functionality didn’t exist at the time this was written. We’ve slowly been migrating.

All that said, that isn’t the correct way to turbofish Into, and I think in such cases the compiler can infer the proper type.

[newpavlov]

Fixed in #1088