Implement object literal rest/spread (...)

Implement object literal rest/spread (...)

As discussed in #245, this syntax is not part of ES6/2015, but it is
a stage 2 proposal, meaning the committee expects it to be included
in the standard.  It's already seeing heavy use by e.g. the React
community.

This change causes `...expr` in an object literal to be parsed as a
unary-node element of an object-node, and modifies places where an
element of an object-node is expected to be an infix-node.  Note that
only weak typing assumptions are made already about object-node elems.

In addition, this change implements proper name extraction in
object literal destructuring.  The cases are:

1. `let {a} = x;`
2. `let {a: b} = x;` (should define `b`)
3. `let {a = b} = x;` (should define `a`)
4. `let {...a} = x;` (should define `a`)

This change fixes case (2), which was broken, and adds case (4).