Get rid of eval

Fix JuliaCollections#48, at the cost of putting the variable in a poorly-performing global.

Not sure if this is acceptable. It's frustrating that Julia seemingly lacks the tools to deal with
this elegantly.

- If `const` worked in a local function, we'd just put `const` and be done with it.
- If `typeconst` existed for global variables, that would work too.

Memoization.jl uses generated functions, which causes other problems. And it feels like the
wrong solution too.

src/Memoize.jl

@@ -53,9 +53,6 @@ macro memoize(args...)
 
     fcachename = cache_name(f)
     mod = __module__
-    fcache = isdefined(mod, fcachename) ?
-             getfield(mod, fcachename) :
-             Core.eval(mod, :(const $fcachename = $cache_dict))
 
     body = quote
         get!($fcache, ($(tup...),)) do
@@ -72,8 +69,10 @@ macro memoize(args...)
     end
 
     esc(quote
+        $fcachename = $cache_dict  # this should be `const` for performance, but then this
+                                   # fails the local-function cache test.
         $(combinedef(def_dict_unmemoized))
-        empty!($fcache)
+        empty!($fcachename)
         Base.@__doc__ $(combinedef(def_dict))
     end)