Support `&str` for `number::complete::hex_u32`

[nipunn1313]

Prerequisites

• Rust version : rustc 1.56.0-nightly (6d64f7f69 2021-08-19)
• nom version : 7.0.0
• nom compilation features used: (default)

Test case

use nom::{
  IResult,
  number::complete::hex_u32,
};

fn hex_u32_on_str() {
    let result: IResult<_, _> = hex_u32("a1");
    println!("{:?}", result.unwrap())
}

Ideally, I'd like this to compile (or something close).

Currently, we get

error[E0308]: mismatched types
  --> src/protocol/parser.rs:17:41
   |
17 |     let result: IResult<_, _> = hex_u32("a1");
   |                                         ^^^^ expected slice `[u8]`, found `str`
   |
   = note: expected reference `&[u8]`
              found reference `&'static str`
help: consider adding a leading `b`
   |
17 |     let result: IResult<_, _> = hex_u32(b"a1");
   |                                         +

Many of the crate::number::complete methods do support both &[u8] and &str, so it seems viable here.