Rename `BlobTree -> FileTree`

[c42f]

While BlobTree/Blob are "technically correct" names they're not very familiar or intuitive.

Some possible renaming options:

• FileTree/File — Pretty good, except clashes with the somewhat unrelated package FileTrees.jl
• Directory/File — Not bad, but this isn't actually a filesystem abstraction
• Dir/File — Dir seems a bit short. People shouldn't need to type this often
• Folder/File — technically correct, but less symmetry and doesn't suggest hierarchy

Having discussed this with a few people, we came to the conclusion that FileTree/File is the most transparent option. The naming clash is unfortunate, but I think we can put up with it for the benefits. (In any case, DataSets.File will clash with FileTrees.File regardless, unless we called it something else. But there's no real obvious alternative to File.)