Another round of polishing (#101)

* Implement basic refactorings

* Simplify some match statements

* Rename `ix` to `index`

If we're not going with a single letter name,
then a full word makes more sense.

* Rename `as_hex` to `to_hex`

`as_` implies lossless reinterpretation
while the function does a conversion that loses information

* Replace `for_each` with for loops

for loops are a lot easier to read and maintain.

* factor out x and y coords in Line::render

this is arguably more ergonomic

* Remove redundant `format!(format_args!())`

core/document/src/document.rs

@@ -1,7 +1,5 @@
-use layers::PolyLine;
-
 use crate::{
-	layers::{self, Folder, Layer, LayerData, LayerDataTypes, Line, Rect, Shape},
+	layers::{self, Folder, Layer, LayerData, LayerDataTypes, Line, PolyLine, Rect, Shape},
 	response::{LayerPanelEntry, LayerType},
 	DocumentError, DocumentResponse, LayerId, Operation,
 };