With the parse.y parser, when a fifo (named pipe) is passed to Kernel#load and friends, we wait for data to be available first before reading. Note that with fifos, opening with `O_RDONLY|O_NONBLOCK` and then reading will look like EOF with read(2) returning 0, but data can become available later. The prism compiler needs to match this behavior to pass `test_loading_fifo_{fd_,threading_raise,threading_success}`. I chose to use IO#read to do this. An alternative way to match behavior would be to use open_load_file() from ruby.c like parse.y, but I opted to only allocate an IO to deal with threading when reading from pipes and character devices. The memory mapping code seems to work fine for regular files.