Fix #202 change EOV to 0xFE

[Shougo]

I created it for #202.
@k-takata @foxundermoon Please test it.

[k-takata]

Changing EOV value breaks binary compatibility. So, it might be a chance to change the data format.
IIRC, we cannot send EOV value itself. How about prepending data size before each data?

+------------+--------+------------+--------+-----+------------+--------+-----+
| datasize 1 | data 1 | datasize 2 | data 2 | ... | datasize N | data N | NUL | 
+------------+--------+------------+--------+-----+------------+--------+-----+

(Data size should be encoded so that it doesn't contain NUL bytes.)

[Shougo]

Changing EOV value breaks binary compatibility. So, it might be a chance to change the data format.
IIRC, we cannot send EOV value itself. How about prepending data size before each data?

OK. Can you implement it?

[k-takata]

Concept model of the new data encoder/decoder in Vim script:
https://gist.github.com/k-takata/37dd48139aff9dfdc4bd

[Shougo]

@k-takata Wow! It is great.

[Shougo]

Closing by #206.