Hierarchical token Bucket

This crate implements Hierarchical Token Bucket algorithm with fixed structure https://en.wikipedia.org/wiki/Token_bucket#Hierarchical_token_bucket

Crate does not rely on periodic updates to maintain the token buckets which means it can be updated right before requesting tokens

```rust use htb::*; use std::time::Duration;

[derive(Clone, Copy, Eq, PartialEq)]

enum Rate { Long, Short, }

impl From for usize { fn from(rate: Rate) -> Self { match rate { Rate::Long => 0, Rate::Short => 1, } } }

// let's implement a rate limiter with two required properties: // - packet rate should not exceed 250 msg per second // - packet rate should not exceed 1500 msg per 15 seconds

let mut htb = HTB::new(&[ BucketCfg { this: Rate::Long, parent: None, rate: (1500, Duration::fromsecs(15)), capacity: 0, }, BucketCfg { this: Rate::Short, parent: Some(Rate::Long), rate: (250, Duration::fromsecs(1)), capacity: 250, }, ])?;

// we are allowed a single 250 token burst assert!(htb.taken(Rate::Short, 250)); assert!(!htb.peek(Rate::Short)); htb.advance(Duration::fromsecs(1));

// after this point established packet rate obeys "long" indefinitely for _ in 0..10 { assert!(htb.taken(Rate::Short, 100)); assert!(!htb.peek(Rate::Short)); htb.advance(Duration::fromsecs(1)); }

// if we stop consuming tokens for some time htb.advance(Duration::fromsecs(10)); assert!(htb.taken(Rate::Short, 250)); // we get more bursts assert!(!htb.peek(Rate::Short));

Ok::<(), Error>(())

```