Rollup merge of #136511 - joshtriplett:nonzero-cast-signed-unsigned, r=dtolnay

Add `cast_signed` and `cast_unsigned` methods for `NonZero` types

Requested in https://github.com/rust-lang/rust/issues/125882 .

Note that this keeps the same names as the methods currently present on other
integer types. If we want to rename them, we can rename them all at the same
time.