Impl Add, Sub for block::Height

[hdevalence]

Is your feature request related to a problem? Please describe.

It would be nice to be able to add and subtract block heights. However, block heights themselves are heights, so adding and subtracting heights doesn't give a new height, it gives a difference between heights: block heights are a torsor. The ops impls we add should reflect this.

Describe the solution you'd like

We should add the following impls for block::Height:

impl Sub<Height> for Height {
    type Output = i32;
    /* ... */
}

impl Add<i32> for Height {
    type Output = Option<Height> // None if over/under max/min height
    /* ... */
}

impl Sub<i32> for Height {
    type Output = Option<Height> // None if over/under max/min height
    /* ... */
}

Describe alternatives you've considered

• Implementing addition/subtraction for heights themselves -- this isn't a quite correct modeling of the problem, and trying to do this probably indicates a logic error;

• Adding a custom newtype for height differences -- this provides little type safety compared to using an i32, at the cost of adding and maintaining another new type, meaning additional API surface, additional impls to write, code to maintain, etc.

[oxarbitrage]

Will be working on this one.

[yaahc]

We should add the following impls for block::Height:

I'm assuming all the i32's in these signatures are meant to represent the diffs, my preference would be to create a HeightDiff newtype to make this more clear.