[First Draft] Add @gc as a Function Attribute

[Walter Bright]

Most of your post was about @safe/@trusted/@system. I had neglected to mention 
that @default wouldn't apply to that, as there is no purpose to it for those 
attributes.

@default would only apply to nothrow, pure, @nogc. It would not override inference.

For example,

@nogc @default = as if @nogc never happened

@default = no effect

@default @nogc = @nogc

and so on.

This addresses your issues.