internal/timeseries: New package.

This implements a time series data structure.
It is not a general purpose package, but will be used
by the upcoming trace package.

Change-Id: I3aa547b2f76582fea246b2b35b465b35499f3fda
Reviewed-on: https://go-review.googlesource.com/10720
Reviewed-by: Andrew Gerrand <adg@golang.org>