Tuples, CTFE, and Sliding Template Arguments

[Richard (Rikki) Andrew Cattermole]

I started working on an alternative mechanism to templates for 1036e 
yesterday. Which I alluded to.

Being able to pass a UDA at a call site and be able to get access to it 
via ``__traits(getAttributes, parameter)`` within the (templated) function.

``i"$ident$(expr)$(ident:format)${1:format}"``

becomes

```d
@IExpression("ident")
ident,
@IExpression("expr")
expr,
@IExpression("ident")
@IFormat("format")
ident,
@IFormat("format")
@IPosition(1)
IPosition.init
```

This would be a general purpose language feature.

```d
string username, password;

getopt(
     @description("My program")
     @description("Second line")
     commandsInfo,
     @description("My programs help info")
     @flag("help") @flag("h") helpInfo,
     @description("The username to connect with")
     @flag("username") @flag("u") username,
     @description("The password to connect with")
     @flag("password") @flag("p") password
);
```

I have already mocked up the getopt, the only additional template usage 
is for formattedRead. This is a general purpose feature, that string 
interpolation could tie into.

https://gist.github.com/rikkimax/812de12e600a070fe267f3bdc1bb3928