Split serve_dir.rs into multiple files

[davidpdrsn]

serve_dir.rs was getting quite big (1.4k lines) so thought it made sense to break up. This breaks the code into these modules:

• future: Contains the response future and response generation code
• headers: Contains some newtypes for a few headers. Was previously defined in the fs module but only used in serve_dir
• open_file: Contains code for opening the file or possibly returning a response early
• tests: Contains, well, the tests

Other than moving code around I made very few changes. The biggest one is renaming Output to OpenFileOutput and making the enum variants more specific. For example previously it contained a StatusCode variant which I replaced with explicit NotFound, PreconditionFailed, and NotModified variants.

[Nehliin]

Nice