Make threaded the second type parameter to reduce typing

Author: penelopeysm

src/model.jl

@@ -1,5 +1,5 @@
 """
-    struct Model{F,argnames,defaultnames,missings,Targs,Tdefaults,Ctx<:AbstractContext,Threaded}
+    struct Model{F,Threaded,argnames,defaultnames,missings,Targs,Tdefaults,Ctx<:AbstractContext}
         f::F
         args::NamedTuple{argnames,Targs}
         defaults::NamedTuple{defaultnames,Tdefaults}
@@ -38,7 +38,7 @@ Model{typeof(f),(:x, :y),(:x,),(:y,),Tuple{Float64,Float64},Tuple{Int64}}(f, (x
 ```
 """
 struct Model{
-    F,argnames,defaultnames,missings,Targs,Tdefaults,Ctx<:AbstractContext,Threaded
+    F,Threaded,argnames,defaultnames,missings,Targs,Tdefaults,Ctx<:AbstractContext
 } <: AbstractProbabilisticProgram
     f::F
     args::NamedTuple{argnames,Targs}
@@ -56,8 +56,8 @@ struct Model{
         args::NamedTuple{argnames,Targs},
         defaults::NamedTuple{defaultnames,Tdefaults},
         context::Ctx=DefaultContext(),
-    ) where {missings,F,argnames,Targs,defaultnames,Tdefaults,Ctx,Threaded}
-        return new{F,argnames,defaultnames,missings,Targs,Tdefaults,Ctx,Threaded}(
+    ) where {Threaded,missings,F,argnames,Targs,defaultnames,Tdefaults,Ctx}
+        return new{F,Threaded,argnames,defaultnames,missings,Targs,Tdefaults,Ctx}(
             f, args, defaults, context
         )
     end
@@ -94,11 +94,12 @@ function Model{Threaded}(
     return Model{Threaded}(f, args, NamedTuple(kwargs), context)
 end
 
-function _requires_threadsafe(
-    ::Model{F,A,D,M,Ta,Td,Ctx,Threaded}
-) where {F,A,D,M,Ta,Td,Ctx,Threaded}
-    return Threaded
-end
+"""
+    _requires_threadsafe(model::Model{F,Threaded}) where {F,Threaded}
+
+Return whether `model` has been marked as needing threadsafe evaluation.
+"""
+_requires_threadsafe(::Model{F,Threaded}) where {F,Threaded} = Threaded
 
 """
     contextualize(model::Model, context::AbstractContext)