Add History as an alias for TimeSeries

[stringertheory]

Summary

The name TimeSeries is kind of annoying to type, and often doesn't seem to capture exactly what the intent is when writing the code. The core mental model of this library is "a history of a value over time" — and the class name should reflect that.

History immediately communicates what the object represents:

alice.level = History(default="unknown")
alice.level[date(2015, 3, 1)] = "junior"
alice.level[date(2017, 6, 15)] = "senior"
alice.level[date(2018, 9, 1)]  # "senior"

Proposal

• Add History as an alias for TimeSeries in traces/__init__.py
• Make History the primary name in documentation and examples going forward
• Keep TimeSeries as a working alias indefinitely (possibly deprecate in a future major version)

traces itself is also kind of "meh", but probably the class name matters more than the library name — it's what appears in every line of user code and it's what teaches the mental model.