calc_range_start to cw0

[ethanfrey]

We use this a lot of places. Often with HumanAddr. Sometimes with String or u64.

It would be nice to move the simple helpers for &[u8] like data (Vec<u8>, String, CanonicalAddr, HumanAddr) into cw0 so we don't cut and paste so much between contracts for the standard idiom.

I don't think let limit = limit.unwrap_or(DEFAULT_LIMIT).min(MAX_LIMIT) as usize; would benefit from a function, as it could only obscure the logic, and possibly lead to DEFAULT / MAX mixups.