Allow arguments to String#to_proc result

Among other things, this allows binary operators to work:

```ruby
  (:+).to_proc.call(1, 2)  # => 3
```

This is most commonly encountered as a result of:

```ruby
  [1, 2, 3].reduce(&:+) # => 6
```

(Note the ampersand: not using `inject`'s special-case Symbol handling.)

I originally implemented this by extending the JS escape:

```ruby
  def to_proc
    %x{
      return function(recv) {
        var meth = recv['$' + self];
        var rest = $slice.call(arguments, 1);
        return meth ? meth.apply(recv, rest)
                    : recv.$method_missing.apply(recv, [self].concat(rest));
      };
    }
  end
```

Then I noticed the compiler does a damn-reasonable job at this, and it doesn't require scattering another `send` and `method_missing` dispatch around.