make ObjectCache typestable

src/objectcache.jl

 export ObjectCache
 
 
-struct ObjectCache{T_Key,T_Object}
+struct ObjectCache{T_Key,T_Object,T_Blueprint}
   cache::Dict{T_Key,T_Object}
-  blueprint::Function
+  blueprint::T_Blueprint
 
-  function ObjectCache{T_Key,T_Object}(blueprint::Function) where {T_Key,T_Object}
+  function ObjectCache{T_Key,T_Object}(blueprint::T_Blueprint) where {T_Key,T_Object,T_Blueprint}
     @assert length(methods(blueprint, [T_Key])) > 0 """
       `blueprint` must accept argument `key::$T_Key`
     """
-    return new{T_Key,T_Object}(Dict{T_Key,T_Object}(), blueprint)
+    return new{T_Key,T_Object,T_Blueprint}(Dict{T_Key,T_Object}(), blueprint)
   end
 end
 
 
-function Base.getindex(c::ObjectCache{T_Key,T_Object}, key::T_Key) where {T_Key,T_Object}
+function Base.getindex(c::ObjectCache{T_Key,T_Object,T_Blueprint}, key::T_Key) where {T_Key,T_Object,T_Blueprint}
   obj = get(c.cache, key, nothing)
   !isnothing(obj) && return obj
   obj = c.blueprint(key)::T_Object