zero-saturating size sub

[m4rch3n1ng]

this pr is two commits

• in [b5b9410] i impl Sub for Size, that just mirrors what the SubAssign already does

• [ecb3f85] might be a little controvertial: Sub and SubAssign operations on Size are already saturating, but they saturate to i32::MIN, which i don't personally think is very useful, because a Size can (should) not be negative, which means that i always have to check before subtracting two sizes. i changed it so they saturate to 0.

to saturate to 0, i added an assossiated const ZERO: Self to the Coordinate trait.

if you prefer, i could move them to a Size::saturating_sub(other: Size) instead, but i think that this change makes Size more convenient to use.

[Drakulix]

I can't find any reason against this, given we already had the debug_assert.

@cmeissl @ids1024 @PolyMeilex @i509VCB if there aren't any complains in the next couple of days, I am going to merge this.

[cmeissl]

LGTM

[Drakulix]

Thanks @m4rch3n1ng!