Rust Scrypt deprecation

[mersinvald]

This PR aims to replace rust-scrypt crate with the pure rust scrypt crate implementation for all platforms.

@r8d8 is there a reason to maintain rust-scrypt and not do this transition on unix?

[mersinvald]

Ran the banchmarks, encoding takes forever with both crates somehow.

Performance

scrypt

decode: 2,088,595 ns/iter (+/- 6,930)
encode: Time Out

ryst-scrypt

decode: 2,149,693 ns/iter (+/- 9,936)
encode: Time Out

[r8d8]

rust built-in benchmark was giving unreal results.
The most accurate real-life bench I had, is measurements of generation time for account through emerald-cli interface.

[mersinvald]

@r8d8 I've made custom bench for that, built-in benches couldn't chew through encoding.

Performance

scrypt

decrypt: 2090547 ns/iter
encrypt: 1279643 us/iter

rust-scrypt

decrypt: 2150338 ns/iter
encrypt: 1326455 us/iter

Though the difference is not very noticeable, scrypt is a bit faster then rust-scrypt

UPD: in debug builds encryption takes 150+ seconds, should we consider opting-in the rust-scrypt for debug builds only or postponing this change at all until the rust-lang/cargo#1359 and rust-lang/rust#48683 are resolved?

Do not merge this until #291 is merged, this PR depends on #291