Consider default BSize<T> with T = usize?

[tisonkun]

So far, we have

bsize/bsize/src/types.rs

Lines 20 to 22 in 841a1de

	/// Byte size representation.
	#[derive(Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
	pub struct BSize<T: Unsigned>(pub T);

I'm considering whether it is desired to default:

- pub struct BSize<T: Unsigned>(pub T); 
+ pub struct BSize<T: Unsigned = usize>(pub T); 

Explicitly is good, and I doubt whether usize is the right default here. So I'm not implementing it right now.

Open for discussion and feedback.