JS-bridged value types' immutability is painful

[andymatuschak]

JavaScript doesn't have value types, but we have custom types for things like Point, Rect, Border, Segment, etc. This is painful for a number of reasons, but for this issue, my complaint is that you can't write:

layer.border.width = 3

Instead you have to write:

layer.border.width = new Border({width: 3, color: layer.border.color})

We've intentionally disabled property setters for these value types because their parent wouldn't notice if the properties' values change. We could set up explicit observers of the returned bridged values to fix that.

/cc @saniul (c.f. #70)