Externalize core modules

[klange]

Several modules in the core interpreter would be better implemented as loadable modules:

• [x] os has lots of bindings that bloat the size of the core interpreter and is not necessary or useful in a number of environments.
• [x] stat similarly is just part of the os module...
• [x] time is pretty simple
• [x] dis, while somewhat intertwined with the debugger, could still be separated to keep instruction bindings out of the core.
• [x] fileio could probably be split off
• [ ] threading has some parts that the core depends on, but might still be feasible to separate

[klange]

fileio provides classes that are used in the core VM, so I don't think it can be extracted without extra effort. threading also doesn't seem worth it.

os, stat, time, and dis have all been successfully extracted.