Add return type to signature

[oxinabox]

This is a follow up for once #1 is complete.
At least of of now it never generates the return type.

But it could, it would requires some inspecting of the Code Lowered.

julia> f(x)::Int = round(Int, x)
f (generic function with 1 method)

julia> meth = first(methods(f))
f(x) in Main at REPL[16]:1

julia> Base.uncompressed_ast(meth)
CodeInfo(
    @ REPL[16]:1 within `f'
1 ─ %1 = Main.Int
│   %2 = Main.round(Main.Int, x)
│   %3 = Base.convert(%1, %2)
│   %4 = Core.typeassert(%3, %1)
└──      return %4
)
``