Fix inference for rowhash()

The fact that the type of the columns isn't known at compile time appears
to confuse inference, which isn't able to detect that this function always returns
UInt. This reduces a lot the number of allocations when calling join(), but
doesn't really change performance.

Author: nalimilan

src/datatablerow/datatablerow.jl

@@ -52,7 +52,7 @@ end
 
 # hash of DataTable rows based on its values
 # so that duplicate rows would have the same hash
-function rowhash(dt::DataTable, r::Int, h::UInt = zero(UInt))
+function rowhash(dt::DataTable, r::Int, h::UInt = zero(UInt))::UInt
     @inbounds for col in columns(dt)
         h = hash_colel(col, r, h)
     end