Fix #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.

at the global scope

It makes the macro expansion much more palatable