Provide a proper interface for in-process trace analysis

[semistrict]

There are a bunch of methods in trace package that aren't necessary for most users to know about. Currently they are only used by the z-Pages (ReportSpansByXXX). I'm going to try to hide these without moving around too much code so that the godoc for the trace package becomes more readable but this is generally useful functionality so we should try to figure out a way to expose this more cleanly.

[semistrict]

/cc @bogdandrutu

[bogdandrutu]

census-instrumentation/opencensus-java#981

[bogdandrutu]

Related to this but for Java API.