Make it possible to replace the TraceID/SpanID generation algorithms

[semistrict]

To support special formats as required by some backends (XRay) we should make the TraceID/SpanID algorithms pluggable.

See: census-instrumentation/opencensus-specs#64

[odeke-em]

Am not sure what @bogdandrutu will think of this change but the last time I spoke with him, there was a reservation towards allowing this.

[semistrict]

I think his concern was with replacing the algorithm entirely (which was my original suggestion). This is a more gradual change that users can opt in to.

[rakyll]

We briefly chatted about this issue to depend on NewSpanWithRemoteParent with custom ID generation. Btw, Bogdan's concern was not to introduce a new generator.