sect_files.xxl

"
General file stuff:

`fname File.exists`

`fname File.dirname`

`fname File.path`

`fname File.get`

`"hello world" File.set "file.txt"`. File.set works only with strings. This is
for writing text files.

XXL data files (XD):

These work great with vectors, but are compatible (and slower) with lists.

```
1024*1024*50 count Xd.set "/tmp/mil.xd"
```

Data is written directly with a small header. There is no unnecessary padding
or boxing on vector data.

Performance notes:

As of March 2016, I can write 50 million ints to disk with Xd in 2 seconds. I
can read them back in 0.4sec.  This file is almost perfectly efficient at
201MB. I think that is sufficient for now.