make Easy parametric for callback specialization

Author: StefanKarpinski

src/Curl/Easy.jl

@@ -1,10 +1,10 @@
-mutable struct Easy
+mutable struct Easy{Input<:IO, Output<:IO, Progress<:Function}
     handle   :: Ptr{Cvoid}
-    input    :: IO
+    input    :: Input
     done     :: Threads.Event
     seeker   :: Union{Function,Nothing}
-    output   :: IO
-    progress :: Function
+    output   :: Output
+    progress :: Progress
     req_hdrs :: Ptr{curl_slist_t}
     res_hdrs :: Vector{String}
     code     :: CURLcode
@@ -16,13 +16,14 @@ function Easy(
     output   :: IO,
     progress :: Union{Function,Nothing},
 )
-    easy = Easy(
+    progress = something(progress, (_, _, _, _) -> nothing)
+    easy = Easy{typeof(input), typeof(output), typeof(progress)}(
         curl_easy_init(),
         input,
         Threads.Event(),
         nothing,
         output,
-        something(progress, (_, _, _, _) -> nothing),
+        progress,
         C_NULL,
         String[],
         typemax(CURLcode),