Add points iterators for primitives

[rfuest]

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PR description

This PR adds Primitive::points which returns an iterator over all points inside a closed shape or on a line.

To keep the changes in this PR small the new iterators are mostly copies of the existing ones without the styling features. Future PRs can reduce the code duplication, but some of these changes depend on other features that aren't implemented yet.

[jamwaffles]

This PR looks good to me, but can you talk about about how it will support #182 (comment)? I'd just like to get a picture of where this is headed.

[rfuest]

The points iterators aren't directly influenced by a PrimitiveStyle because they work on "raw" Primitives and not styled primitives. The same behavior will also replicated by the hit testing function, which will essentially return true if a point is contained in primitive::points().

To make it easier to use the same functions for styled primitives additional functions can be added. For example, Rectangle::stroke_area(&self) -> Rectangle and Rectangle::fill_area(&self) -> Rectangle, would return offset versions of the original rectangle that define the outer edge of the styled primitives fill and stroke. Having this functions would make it easy to implement custom drawing routines:

let c = Circle::new(...).into_styled(...);
let stroke_area = c.stroke_area();
let fill_area = c.fill_area();
stroke_area.points()
   .map(|p| if fill_area.contains(p) { FILL_COLOR }  else { STROKE_COLOR })
   .draw(&mut display);

[jamwaffles]

For example, Rectangle::stroke_area(&self) -> Rectangle and Rectangle::fill_area(&self) -> Rectangle, would return offset versions of the original rectangle that define the outer edge of the styled primitives fill and stroke.

That's really elegant, I like it :)

[jamwaffles]

PR looks good to me, although I'm on Windows atm so haven't tested it.