Add support for some big number types

[AkiraMiyakoda]

This PR enables redis-rs to read/write some third-party "big number" types directly.

Motivation

We can store a number with any number of digits in Redis as a string. Although we can read such a value as a string and convert it into a desired numeric type, it's convenient when we can read it directly as a number like this:

use num_bigint::BigInt;

let v: BigInt = connection.get("foo").await?;

This library already works with Rust's intrinsic numeric types. This PR adds a feature called bignum to it so that it can handle some 'big number' types from some third-party crates which are mature and widely used.

What is this?

This PR contains FromRedisValue and ToRedisArgs implementation for following types:

• rust_decimal::Decimal
• bigdecimal::BigDecimal
• num_bigint::BigInt
• num_bigint::BigUint

It includes tests and a feature flag as well.

[nihohit]

This looks mostly OK to me.
@jaymell since these are separate crates, do you think that these should be added as a single feature, or split between multiple features?

[nihohit]

@jaymell LGMT, just not sure whether we want this as a single feature, or split the feature between crates, so that each bignum crate will have its own feature. WDYT?

[jaymell]

This is great! But I definitely think it makes sense to not force everything into one feature. @AkiraMiyakoda Can we split this into multiple features, one for each external crate?

[AkiraMiyakoda]

Hi @jaymell
Thanks for the review. I split the bignum feature into three which correspond to target crates.
New features' names are rust_decimal, bigdecimal and num-bigint just the same as the crates.

[nihohit]

Thanks for the contribution, and the excellent work!