Implement From<bool> for ValueRef

[rhysd]

Value implements From<bool> so we can pass booleans to functions like below.

fn send_some_value(value: impl Into<Value>) {
  // ...
}

send_some_value(true);

However ValueRef doesn't implement it.

fn send_some_value_ref(value: impl Into<ValueRef<'_>>) {
  // ...
}

send_some_value_ref(true); // ERROR

I believe there is no reason not to implement it so I created this PR to add the conversion.