Allow object as string

[gfx]

object as string is useful in cases the object overrides toString() and/or the type of the object is specified by a type parameter (e.g. function f.<T>(value : T) : void { log "label: " + value as T; }).

[kazuho]

Thank you for the work. I am mostly fine with the idea.

But I wonder what happens when both this change and the auto-stringification in + (concatenation operator) gets introduced.

The discussion about the auto-stringification was that as string in the following snippet should be omittable.

var s = "abc";
var n = 123;
var ret = s + n;

And if we add the feature together with this change, it would mean auto-stringification would occur for any type of data. Do you think it is affordable?