Add new() method to Bit trait

[jerry73204]

This PR moves new() from B0 and B1 to Bit trait.
It lets the methods with Bit on return type generics to complete by calling Bit::new().

[paholg]

Seems reasonable, except that removing the old impls is technically a breaking change (the trait might not be in scope).

Since there's no harm in having both, would you mind re-adding B0::new() and B1::new()?

[jerry73204]

I add the new() back and add a test to ensure both of them work.

[paholg]

Sorry for the delay; thanks!