Add syntax for passing a variable as a keyword argument of the same name

[evhub]

Should compile (something like)

func(long_name_1=, long_name_2=)

to

func(long_name_1=long_name_1, long_name_2=long_name_2)

Alternative syntaxes:

func(=long_name_1, =long_name_2)
func(...=long_name_1, ...=long_name_2)

[evhub]

Whatever syntax we pick here should also be supported in partial application and anonymous namedtuples.

[evhub]

The func(...=long_name_1, ...=long_name_2) syntax is now live on coconut-develop>=3.0.0-post_dev7.