Yield at an interval during the loading of directory entries as well as
while sorting them in order to not starve the poll functions.
The maximum time between calls to `user_io_poll` in the main loop were
measured while loading a folder containing 29026 entries:
- Original main-loop: 591ms
- With coroutines + yields: 29ms
The amount of time it takes to load and sort the 29026 entries:
- Original main-loop: 591ms
- With coroutines + yields: 686ms
By increasing the value of `YieldIterations` in `file_io.cpp` the load
time will decrease while increasing the delay between calls to
`user_io_poll`.
Stores directory entries in a std::vector instead of an array of
hardcoded size. The vector will dynamically grow to the number of
available entries in a folder without having a hardcoded upper limit.
