Make the _Message API require using Ref{_Message}'s

Using plain _Message's with most of these functions is likely incorrect.

src/_message.jl

@@ -5,7 +5,7 @@
 # zmq_msg_t in the header: char _[64];
 const _Message = lib.zmq_msg_t
 
-const _MessageOrRef = Union{_Message,Base.RefValue{_Message}}
+const _MessageRef = Base.RefValue{_Message}
 
 function msg_init()
     zmsg = Ref{_Message}()
@@ -22,15 +22,14 @@ function msg_init(nbytes::Int)
 end
 
 # note: no finalizer for _Message, so we need to call close manually!
-function Base.close(zmsg::_MessageOrRef)
+function Base.close(zmsg::_MessageRef)
     rc = lib.zmq_msg_close(zmsg)
     rc != 0 && throw(StateError(jl_zmq_error_str()))
     return nothing
 end
 
-Base.length(zmsg::_MessageOrRef) = lib.zmq_msg_size(zmsg) % Int
-Base.unsafe_convert(::Type{Ptr{UInt8}}, zmsg::_Message) = Ptr{UInt8}(lib.zmq_msg_data(Ref(zmsg)))
-Base.unsafe_convert(::Type{Ptr{UInt8}}, zmsg::Base.RefValue{_Message}) = Ptr{UInt8}(lib.zmq_msg_data(zmsg))
+Base.length(zmsg::Base.RefValue{_Message}) = lib.zmq_msg_size(zmsg) % Int
+Base.unsafe_convert(::Type{Ptr{UInt8}}, zmsg::_MessageRef) = Ptr{UInt8}(lib.zmq_msg_data(zmsg))
 
 # isbits data, vectors thereof, and strings can be converted to/from _Message
 
@@ -57,7 +56,7 @@ function _MessageRef(x::String)
     return zmsg
 end
 
-function unsafe_copy(::Type{Vector{T}}, zmsg::_MessageOrRef) where {T}
+function unsafe_copy(::Type{Vector{T}}, zmsg::_MessageRef) where {T}
     isbitstype(T) || throw(MethodError(unsafe_copy, (T, zmsg,)))
     n = length(zmsg)
     len, remainder = divrem(n, sizeof(T))
@@ -67,16 +66,16 @@ function unsafe_copy(::Type{Vector{T}}, zmsg::_MessageOrRef) where {T}
     return a
 end
 
-function unsafe_copy(::Type{T}, zmsg::_MessageOrRef) where {T}
+function unsafe_copy(::Type{T}, zmsg::_MessageRef) where {T}
     isbitstype(T) || throw(MethodError(unsafe_copy, (T, zmsg,)))
     n = length(zmsg)
     n == sizeof(T) || error("message length $n ≠ sizeof($T)")
     return @preserve zmsg unsafe_load(Ptr{T}(Base.unsafe_convert(Ptr{UInt8}, zmsg)))
 end
 
-function unsafe_copy(::Type{String}, zmsg::_MessageOrRef)
+function unsafe_copy(::Type{String}, zmsg::_MessageRef)
     n = length(zmsg)
     return @preserve zmsg unsafe_string(Base.unsafe_convert(Ptr{UInt8}, zmsg), n)
 end
 
-unsafe_copy(::Type{IOBuffer}, zmsg::_MessageOrRef) = IOBuffer(unsafe_copy(Vector{UInt8}, zmsg))
+unsafe_copy(::Type{IOBuffer}, zmsg::_MessageRef) = IOBuffer(unsafe_copy(Vector{UInt8}, zmsg))