[WIP] Add draft support for binary fixed width files

This pull request starts down the path of supporting reading DataFrames from fixed-width files. The current implementation is a barebones implementation of something like R's `scan()` function. I still need to write a version that reads fixed-width files in which all entries are plain-text and need type inference.

Eventually I'd like to move all of our I/O tools into a separate I/O package. But for now I'd just like to get this tool written.

Please bikeshed the names `readfwfbin` and `writefwfbin`. I don't like the name `scan`, but we can definitely do better than the names I've chosen so far.