Remove double allocations in Base.map by having to reverse the list

instead create it recursively in-order, via tail-recursion

Big perf improvement:

Before:
```julia
julia> @Btime DataStructures.map(x->x*2, $(list((1:1000)...)));
  148.476 μs (6469 allocations: 163.55 KiB)
```
After:
```julia
julia> @Btime DataStructures.map(x->x*2, $(list((1:1000)...)));
  29.705 μs (1745 allocations: 42.89 KiB)
```

src/list.jl

@@ -70,12 +70,8 @@ end
 Base.map(f::Base.Callable, l::Nil) = l
 
 function Base.map(f::Base.Callable, l::Cons{T}) where T
-    first = f(l.head)
-    l2 = cons(first, nil(typeof(first) <: T ? T : typeof(first)))
-    for h in l.tail
-        l2 = cons(f(h), l2)
-    end
-    reverse(l2)
+    rest = Base.map(f, l.tail)
+    return cons(f(l.head), rest)
 end
 
 function Base.filter(f::Function, l::LinkedList{T}) where T