Support Traitor syntax?

[mauro3]

Should Traitor-like syntax sugar be added? Looks like:

@traitfn function howbig(x::Any::Big)
    "Huge!"
end

from the readme of https://github.com/andyferris/Traitor.jl. Would be equivalent to:

@traitfn function howbig{X; Big{X}}(x::X)
    "Huge!"
end

[mauro3]

Note, that still only one trait can appear per function signature.

Thanks to @c42f and @andyferris for the good ideas.