Better macros

[maciejhirsz]

• Improved macro syntax.

---

Instead of:

let obj = object! {
    "foo" => array![1, 2, json::Null],
    "bar" => 42
};

Macros now support following, compact syntax:

let obj = object! {
    "foo": [1, 2, null],
    bar: 42 // quotes on keys are optional
};

This is a backwards compatible change, old macro use will continue to work. Mixing => and : style objects will lead to compile errors, it's either the new style or the old one.

[maciejhirsz]

0.13 is going to introduce major breaking changes, and it might make it easier to manage if all the macros and tests aren't also modified from last stable release.

Since this is a reasonably big feature, but it is still mostly surface quality of life stuff, and it's backwards compatible, gonna roll this out as 0.12.3.