replace usize with u64

[alexanderkjall]

as usize is 32-bit on 32-bit architectures.

Currently the project doesn't build on any 32-bit architecture: https://buildd.debian.org/status/package.php?p=rust-compound-duration

With problems like this:

warning: this arithmetic operation will overflow
   --> src/lib.rs:169:26
    |
169 |     let mut ns = nanos % (DAY * NANOS);
    |                          ^^^^^^^^^^^^^ attempt to compute `86400_usize * 1000000000_usize`, which would overflow
    |
    = note: `#[warn(arithmetic_overflow)]` on by default

It looks to me like this breaks your public api, so I also bumped the major version to 2.