`#[inline]` integer parsing functions

This improves the performance of `str::parse` into integers.

Before:
```
compare          fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ std                         │               │               │               │         │
   ├─ 328920585  10.23 ns      │ 24.8 ns       │ 10.34 ns      │ 10.48 ns      │ 100     │ 25600
   ├─ 3255       8.551 ns      │ 8.59 ns       │ 8.551 ns      │ 8.56 ns       │ 100     │ 25600
   ╰─ 5          7.847 ns      │ 7.887 ns      │ 7.847 ns      │ 7.853 ns      │ 100     │ 25600
```

After:
```
compare          fastest       │ slowest       │ median        │ mean          │ samples │ iters
╰─ std                         │               │               │               │         │
   ├─ 328920585  8.316 ns      │ 23.7 ns       │ 8.355 ns      │ 8.491 ns      │ 100     │ 25600
   ├─ 3255       4.566 ns      │ 4.588 ns      │ 4.586 ns      │ 4.576 ns      │ 100     │ 51200
   ╰─ 5          2.877 ns      │ 3.697 ns      │ 2.896 ns      │ 2.945 ns      │ 100     │ 102400
```

Benchmark:
```rust
fn std(input: &str) -> Result<u64, ParseIntError> {
    input.parse()
}
```