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 definitely don't think that `convert` should work on `real`. That just makes it easier to operate on time as a floating point value, which I do _not_ want to encourage, and it's not necessary for constructing `Duration`s from floating point values.