Use promotion rather than typejoin to infer map/broadcast output eltype

This allows `Union`s as the output type for mixtures of `Null` and
numeric types, which can be more efficient than the `Any` you'd get from
typejoin, at least in 0.7.

Author: c42f

src/broadcast.jl

@@ -105,7 +105,7 @@ end
     return quote
         @_inline_meta
         elements = tuple($(exprs...))
-        @inbounds return similar_type($first_staticarray, eltype(elements), Size($newsize))(elements)
+        @inbounds return similar_type($first_staticarray, promote_tuple_eltype(elements), Size($newsize))(elements)
     end
 end
 

src/mapreduce.jl

@@ -25,7 +25,7 @@ end
     return quote
         @_inline_meta
         elements = tuple($(exprs...))
-        @inbounds return similar_type(typeof(_first(a...)), eltype(elements), Size(S))(elements)
+        @inbounds return similar_type(typeof(_first(a...)), promote_tuple_eltype(elements), Size(S))(elements)
     end
 end
 
@@ -126,7 +126,7 @@ end
     return quote
         @_inline_meta
         elements = tuple($(exprs...))
-        @inbounds return similar_type(a, eltype(elements), Size($Snew))(elements)
+        @inbounds return similar_type(a, promote_tuple_eltype(elements), Size($Snew))(elements)
     end
 end