object! performance could be improved

[pmnoxx]

I started using object! macro, but I noticed that it's performing poorly, due to unnecessary memory allocations. We can optimize the code, because we know the number of arguments we are going to put into the array.
Here are the results:

test object ... bench: 52,500,999 ns/iter (+/- 1,924,204)
test object_new ... bench: 33,950,158 ns/iter (+/- 685,775)

`
#![feature(test)]

extern crate test;
#[macro_use]
extern crate json;

use json::JsonValue;
use test::Bencher;

macro_rules! count {
() => (0usize);
( $x:tt $($xs:tt)* ) => (1usize + count!($($xs)*));
}

macro_rules! obj {
{} => ($crate::JsonValue::new_object());

{ $( $key:expr => $value:expr ),* } => {
    obj!( $(
        $key => $value,
    )* )
};

{ $( $key:expr => $value:expr, )* } => ({
    use json::object::Object;

    let cnt = count!( $( $key )* );
    let mut object = Object::with_capacity(cnt);
    $(
        object.insert($key, $value.into());
    )*

    JsonValue::Object(object)
})

}

#[bench]
fn object_new(b: &mut Bencher) {
b.iter(|| {
for it in 0..100_000 {
let mut val: JsonValue = obj! {
"data"=> obj!{
"arg1" => 23,
"arg2" => 44,
},
"test"=> 880032432,
"anki"=> JsonValue::Null,
};
}
});
}

#[bench]
fn object(b: &mut Bencher) {
b.iter(|| {
for it in 0..100_000 {
let mut val: JsonValue = object! {
"data"=> obj!{
"arg1" => 23,
"arg2" => 44,
},
"test"=> 880032432,
"anki"=> JsonValue::Null,
};
}
});
}

`