Checking function parameters in Phobos

[Dicebot]

I also think this is very powerful and under-explored approach 
but it really better belongs to certain domain framework than to 
stdlib. One example I keep thinking about is to re-declare vibe.d 
string functions in terms of EscapedString!(SQL), 
EscapedString!(HTML) and so on for better application safety and 
correctness. No idea how that may work in practice though.