split: Initial import

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>

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Here is a split implementation.

I have a subsidiary project for testing (https://github.com/akiradeveloper/rust-split) where I compare my split with GNU split as a reference implementation in a way like QuickCheck.

Quick look at the performance says my split is twice slower than GNU one. Not desperate, and I will improve this later.
```stdout()``` already returns a ```BufferedWriter```.