feat(scrypt): Add Params::n method

[sorairolake]

Currently, the N parameter can be obtained by raising 2 to the power of Params::log_n.

// example of raising 2 to the power of `Params::log_n`
let n = 1 << params.log_n();

I think it's easier to read to use a method which gets the N parameter directly instead of this way.

let n = params.n();