In addition to providing more functionality for creating -b/--by fields,
this lets us remove strings from the expr parser. Strings had no
well-defined operations and could best be described as an "ugly wart".
Maybe we'll reintroduce string exprs in the future, but for now csv.py's
-f/--field fields will be limited to numeric values.
As an extra plus, no more excessive quoting when injecting new -b/--by
fields.
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This also fixed sorting on non-field fields, which was apparently
broken. Or at least mostly useless since it was defaulting to string
sorting.
Christopher Haster
dcbc195b41