Introduce a Merge Function trait

[Kerollmops]

Currently, the merge function is anonymous. It is not very practical when you are refactoring code. I mean, typing thing is much better and makes the work easier. The code would also be less ugly.

grenad/src/merger.rs

Lines 106 to 110 in 3e684da

	impl<R, MF, U> Merger<R, MF>
	where
	R: io::Read + io::Seek,
	MF: for<'a> Fn(&[u8], &[Cow<'a, [u8]>]) -> Result<Cow<'a, [u8]>, U>,
	{