std.templatecons ready for comments

[Dicebot]

(x-post from .announce:)

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 :)