Create root spans allowing for override of Span identifier

[istathar]

Presents an alternative API to that suggested by #125. The idea here is that we would create a trace but without specifying the parent, using a new usingTrace' function. Then we would re-use the existing encloseSpan function augmented by a new override setIdentifierSpan function, resulting in a shape like this:

    usingTrace' trace $ do
        encloseSpan "Eat your vegetables" $ do
            setStartTime start 
            setIdentifierSpan predetermined
            telemetry
                [ metric "sugar" False
                ]

where trace and predetermined are the received and already created Trace and Span identifiers, respectively.

While not quite as compact as the sendSpan idea proposed by @TheWizardTower, I'd like to suggest that by reusing existing encloseSpan machinery we keep the API surface a bit more contained. setStartTime is already an outlier as an override, and so this usage probably makes sense.

[istathar]

Might be worth noting that @TheWizardTower's sendSpan is

sendSpan label traceId spanId serviceName spanId start meta =
    usingTrace' traceId $ encloseSpan label (setIdentifierSpan spanId >> setServiceName serviceName >> setStartTime start >> telemetry meta)