Binary / text file loading

Here's the API.

common.fetch("bin", fname) is meant to return a function, which, when called, returns a string. This is a workaround for the deferred loading system, which works OK for functions, but if we were to do it for strings, we'd end up with a horrible metatable + wrapper function hell, so it's better to just have a __call which returns our intended string.

...that's it, really.