`each{,Default}System{,Map}`: accept binary functions

[ilkecan]

• eachSystem
• eachDefaultSystem
• eachSystemMap
• eachDefaultSystemMap

The functions passed to these 4 functions above can now take an additional argument after their first one (previously system -> attrs, now system -> self -> attrs). The optional self argument can be used to refer to the fixed point of the function (self -> attrs).

Closes: #78

[zimbatm]

sounds good! This just requires a rebase (also sorry for being so late to review)