Main: Split off block I/O

Main: Split off block I/O

Block-based I/O is no longer hard-wired in the mainloop itself,
it became a separate module built on the top of main_file. While the
timer embedded in main_block_io is somewhat illogical, I have decided
to keep it to make porting of old code easier.

I plan to add another I/O layer in the future, which will handle
more complex buffering capable of reading lines and other variable-size
data structures.