Add @unpack decorator

[ADR-007]

I propose to add decorator which will unpack argument:

    def process(foo, bar):
        ...
    
    map(unpack(process), product(['foo1', 'foo2'], ['bar1', 'bar2']))

This decorator already is in ddt library, but it will be
strange if I will install this module only for this.

[Suor]

See discussion in #62.