Add possibility for a type annotation for letFunction

[lue-bird]

letVal and letFunction should allow an added type annotation like funDecl already does for example:

let
    svgAttributeToElm : ( String, String ) -> CodeGen.Expression
    svgAttributeToElm ( name, value ) =
        ...
in
...

On this note, I would rename letFunction to letFun to be more consistent.