Parsing of binary operators in struct field names

[Keno]

The following surprised me:

julia> struct Foo
           +::Function
       end
ERROR: syntax: invalid "::" syntax around REPL[22]:1
Stacktrace:
 [1] top-level scope
   @ REPL[22]:1

Now, I happen to know enough about Julia to know that the following would probably work:

julia> struct Foo
           (+)::Function
       end

but I can imagine many users would not know what to do. If there's nothing wrong with allowing
the first case, I would suggest we might want to consider doing that. Otherwise, it would
be nice to get some sort of hint in the error message to tell the user what to do.

[JeffBezanson]

Parses as a unary call to + on the expression ::Function, which is pretty useless. We could try terminating unary parsing if the next token is ::; will take some fiddling no doubt.