Adds Mixed Integer Ops

[NCGThompson]

This branch implements methods analogous to those of the stable feature mixed_integer_ops.

Closes #24.

[nlordell]

nit: I would write this as:

Suggested change

	/// # use ethnum::I256;
	/// use ethnum::U256;
	/// # use ethnum::{I256, U256};

And same for other occurrences 👇.

[nlordell]

My rationale is that it feels weird to only show one of the use statements in the doc test but not the other.

[NCGThompson]

The majority of the methods only require importing Self, and that import is hidden. I tried to match the patterns of the context as closely as possible. I think since the docs will appear on the Self page, Self's use statement is implied, but Other's use statement is not. There is some precedent in std.

A trilema:

• Hide one use statement but not the other in the same doc test.
• Hide all use statements in one doc test but none in the other doc test.
• Hide/Show all use statements in all doc tests.

[nlordell]

Good point... In my mind there is actually a quadlema:

• Hide all ethnum use statements but not other use statements in the same doc test

Which, while completely arbitrary, kind of makes sense in the context of usage documentation from the ethnum crate's perspective.

I will merge this as is, and think about it a bit more to see if I want to change it or not.

[NCGThompson]

I pushed a commit addressing this right after you merged.

[nlordell]

Thanks - your points were valid though, and I'm still a bit on the fence on whether only Self use statements should be hidden or not.

[nlordell]

Thanks for the contribution!