Clarify tracking of highest and lowest trackable values

See https://github.com/jonhoo/hdrsample/pull/74#discussion_r158192909 for more context.

We save in fields (and write when serializing) the requested limits, not the actual limits that result from the underlying encoding (which will encompass at least as much as what the user requested, and maybe more). Perhaps we should expose the actual limits of what a particular histogram can do, rather than just regurgitate the limits that the user requested? This would be useful when, say, storing metadata about histograms, since the data actually in the histogram is likely more interesting than the particular subset of values that were initially requested as trackable.

Strawman:

• configured_low() for what the user requested when creating the histogram
• actual_low() for what the histogram can support
• configured_high(), actual_high()

Start with the linked pull-request discussion and inspect how the requested limits are stored and written during histogram serialization. Resolve whether configured and actual limits should both be exposed and serialized; the issue does not name a file or test, and its strawman still needs a project decision.