std.templatecons ready for comments

[Dicebot]

Had a very quick run-through the provided functionality. Looks 
nice and has some of utilities I tend to reinvent in my own code 
when doing meta-programming. Though I'd prefer to initiate 
creation of nested `std.meta` package and move such stuff there 
instead of adding yet another plain module. Also I am trying to 
negotiate with Andrei & Walter into deciding std.typetuple fate 
which may impact implementation of such module quite a lot :)