Optimize `FileLoad` by not using fstream.
For some reason fstream is just really bad.
Switching over to raw pread cuts the amount of time it takes to read
files by a quarter of CPU time.
If we have a fixed file to read, we can read it in three syscalls.
fstream is...weirdly slow in the other implementation.
Theoretically in the future this can be improved to a single syscall if
the `readfile` syscall ends up in upstream Linux.
Ryan Houdek