`bits` => `bit_width`

[tarcieri]

We currently provide bits and bits_vartime methods to compute the number of bits needed to represent a particular value.

It seems that Rust 1.97 stabilized <{uN}>::bit_width as the name for the core equivalent of these methods.

We should probably follow suit, switching from bits => bit_width, and deprecating the old name.

(Sidebar: does this make bits_precision inconsistent, or should we keep that?)