refactor(layout): simplify and doc split() (#405)

* test(layout): add tests for split()

* refactor(layout): simplify and doc split()

This is mainly a reduction in density of the code with a goal of
improving mainatainability so that the algorithm is clear.

Cargo.toml

@@ -58,6 +58,7 @@ criterion = { version = "0.5", features = ["html_reports"] }
 fakeit = "1.1"
 itertools = "0.10"
 rand = "0.8"
+pretty_assertions = "1.4.0"
 
 [[bench]]
 name = "block"

src/backend/test.rs

@@ -93,6 +93,7 @@ impl TestBackend {
     /// Asserts that the TestBackend's buffer is equal to the expected buffer.
     /// If the buffers are not equal, a panic occurs with a detailed error message
     /// showing the differences between the expected and actual buffers.
+    #[track_caller]
     pub fn assert_buffer(&self, expected: &Buffer) {
         assert_eq!(expected.area, self.buffer.area);
         let diff = expected.diff(&self.buffer);