feat(socketio/ns): store ns path and rooms as Cow<'static, str>

[Totodore]

Store ns path and rooms as Cow<'static, str> :

• Change ns path from String to Cow
• Change rooms in LocalAdapter from String to Cow
• Change events callbacks in Socket from String to Cow
• Pre-allocate a String buffer with a size_hint function when encoding to avoid string reallocations
• Decode Packets from bytes slice rather than String to avoid Char iterators (removes deps itertools)
• Benchmark memory consumption before and after

[Totodore]

Benchmark results: benchmark-feat-cow-namespaces-and-rooms.zip
Generated with: cargo bench --bench packet_encode --bench packet_decode --all-features

[Totodore]

One of the remaining things that won't be improved in this PR is the "int to string" conversion. It is currently using to_string which allocates.

One of the solution was to use this function, however it inserts digits in reverse order:

fn insert_number(mut v: usize, res: &mut String) {
    debug_assert!(v > 0);
    while v > 0 {
        let n = (v % 10) as u8;
        v /= 10;
        res.push((n + 0x30) as char);
    }
}