budfs support

[matthewmueller]

This is an experimental v3 of the generator filesystem. The goals are:

• Bake in dependency caching so running spawning subprocesses only occurs when dependent files change
• Fix minor issues around dir entries on generated files having a size of 0 or 0 for their file mode
• Join conjure and overlay into one bud-specific filesystem
• Be able to serve files using the same filesystem without caching
• Better errors (conjure => budfs)