Update to 0.6.4

changes:
0.6.4
  - Added Text/TeXMath/Pandoc to repository.
      Closes https://github.com/jgm/texmath/issues/18
  - Added Text.TeXMath.Pandoc module.
      This factors out some of the code formerly in
      Text.Pandoc.Readers.TeXMath.
Import libeio-4.18 as wip/libeio.

Libeio is a full-featured asynchronous I/O library for C,
modelled in similar style and spirit as libev. Features
include: asynchronous read, write, open, close, stat,
unlink, fdatasync, mknod, readdir etc. (basically the full
POSIX API). sendfile (native on solaris, linux, hp-ux,
freebsd, emulated everywehere else), readahead (emulated
where not available).
Add libeio