std::convert::From<usize> cannot be used here because we cannot
implement bool<->usize conversions, due to Rust's orphan rules:
http://smallcultfollowing.com/babysteps/blog/2015/01/14/little-orphan-impls/
"prevent you from implementing external traits for external types"
Nonetheless, Lengthable used the same methods as From. This is allowed
but can require disambiguation if both are used, no longer strictly
needed for #140 but to reduce confusion and improve clarity, renamed
`from` to `from_len` and `into` to `into_len`.
87bfb8d2a4