Define CompoundPeriod conversion/constructor for Real

[charleskawczynski]

This PR defines conversions, or rather, CompoundPeriod constructors that accept Real numbers. For example (from the tests added):

    ccp(x, p) = Dates.canonicalize(Dates.CompoundPeriod(x, p))
    @test string(ccp(1.5, Dates.Minute)) == "1 minute, 30 seconds"
    @test string(ccp((Dates.Second(Dates.Week(1))+Dates.Second(1)).value, Dates.Second)) == "1 week, 1 second"
    @test string(ccp(1.5, Dates.Day)) == "1 day, 12 hours"
    @test string(ccp(1.5+7, Dates.Day)) == "1 week, 1 day, 12 hours"
    @test string(ccp(1.4, Dates.Second)) == "1 second, 400 milliseconds"
    @test string(ccp(10.1, Dates.Week)) == "10 weeks, 16 hours, 48 minutes"
    @test string(ccp(10.1, Dates.Week) - Dates.Minute(6)) == "10 weeks, 16 hours, 42 minutes"

[charleskawczynski]

One downside of this feature is that it gets hit pretty hard by #52873, but I think we can fix that.