Improve JSDoc support for destructured parameters

Currently it is very cumbersome to use destructuring for function parameters with more than a couple properties, particular if many are optional:

``` js
@param {{a: (number|undefined), b: (number|undefined), c: (number|undefined)}} param1
```

Note that the `number=` syntax does not work, and in fact leads to #1762.

Ideally we could write

``` js
/**
@param {number=} a
@param {number=} b
@param {number=} c
*/
function({a = 1, b = 2, c = 3} = {}) {}
```