fix Issue 13176 - core.time should allow conversions of floating-point values too

I've needed this several times. Usually when we ask the user for a time value (e.g. seconds), it's more useful to allow them to specify the value as a fractional number as well - so that e.g. they could tell an utility to have a timeout of 0.5 seconds.

https://issues.dlang.org/show_bug.cgi?id=13176
I don't think that we should make it so that `total` works with floating point values. That does not help the stated use case of being able to do stuff like `Thread.sleep(seconds(1.5))`, and it makes it easier for folks to try and manipulate time as a floating point value, which will just help create bugs.