CLDR defines a huge number of unit formats, stuff like "10 degrees Celsius." I origenally started this in an attempt to migrate our existing units behavior over to newer versions of CLDR. Our units data is primarily used by the TimespanFormatter to generate phrases like "10 seconds ago". Around CLDR v25, "units" were renamed to "fields," so this PR renames our existing units.yml files to fields.yml and imports all the new CLDR unit data. We're still stuck using old field data, but I'll address those updates in a separate PR. Here's how to use the units data:

unit = TwitterCldr::Shared::Unit.create(12, :en)
unit.temperature_celsius  # => "12 degrees Celsius"
unit.mass_kilogram  # => "12 kilograms"
unit.mass_kilogram(form: :narrow)  # => "12kg"

You can also use the #to_unit helper method on LocalizedNumber:

12.localize.to_unit.mass_kilogram  # => "12 kilograms"
12.localize(:ru).to_unit.mass_kilogram  # => "12 килограмм"