Fixes gh-195: parts should be key_fields.

Consistently use parts to refer to parts of a multipart key definition.
Fixes gh-199: required_field_count is better than arity

Weed out word "arity" and use "field_count" instead: in
variable names, error messages, tests, constants.
Merge branch 'master' of https://github.com/tarantool/tarantool