refactor(layout): to natural reading order

[joshka]

Structs and enums at the top of the file helps show the interaction
between the types without having to find each type in between longer
impl sections.

Also moved the try_split function into the Layout impl as an associated
function and inlined the layout::split() which just called try_split.
This makes the code a bit more contained.

---

This is a redo of the refactor step from #641 to avoid losing code in a bad rebase.

For ease of checking that all the methods and structs are reorganized:

grep -E '^\s*(fn|pub|enum|struct|impl)' src/layout.rs | sed 's/^ *//' | sort > layout-refactor-sorted.txt
diff -u layout-main-sorted.txt layout-refactor-sorted.txt

--- layout-main-sorted.txt   2023-12-09 13:18:02
+++ layout-refactor-sorted.txt  2023-12-09 13:19:02
@@ -31,7 +31,6 @@
 fn segment_size_from_string() {
 fn segment_size_to_string() {
 fn size(&self) -> Expression {
-fn split(area: Rect, layout: &Layout) -> Rc<[Rect]> {
 fn test(area: Rect, constraints: &[Constraint], expected: &str) {
 fn test_constraint_apply() {
 fn test_split_equally_in_overconstrained_case_for_length() {
@@ -43,12 +42,12 @@
 impl AsRef<Constraint> for Constraint {
 impl Constraint {
 impl Default for Constraint {
+impl Display for Constraint {
+impl Display for Margin {
 impl Element {
 impl From<(u16, u16)> for Size {
 impl Layout {
 impl Margin {
-impl fmt::Display for Constraint {
-impl fmt::Display for Margin {
 pub const DEFAULT_CACHE_SIZE: usize = 16;
 pub const fn direction(mut self, direction: Direction) -> Layout {
 pub const fn horizontal_margin(mut self, horizontal: u16) -> Layout {

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (03401cd) 90.9% compared to head (45d3e23) 90.9%.

Files	Patch %	Lines
src/layout.rs	97.3%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #681     +/-   ##
=======================================
- Coverage   90.9%   90.9%   -0.1%     
=======================================
  Files         42      42             
  Lines      12678   12677      -1     
=======================================
- Hits       11526   11525      -1     
  Misses      1152    1152             

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.