Make library more usable in a const context (#203)

* Add FlexboxLayout::DEFAULT constant

* Make constructors const where possible

* Use Size::NONE instead of Size::undefined()

* Stick with Layout::new

* More advice on setting up chromedriver

* More const builders

* Update tests

* Fix find-and-replace error in CHANGELOG

CONTRIBUTING.md

@@ -28,6 +28,10 @@ If you are developing on macOS this is easy to do through brew.
 brew install chromedriver
 ```
 
+If you are Ubuntu, you will have to install `openssl` first,
+and then can follow [these instructions](https://tecadmin.net/setup-selenium-chromedriver-on-ubuntu/).
+Be sure that your Chrome vesion matches the downloaded `chromedriver` version!
+
 Once you have chromedriver installed and available in `PATH` you can re-generate all tests by running `cargo run --package gentest`. You should not manually update the tests in `tests/generated`. Instead, fix the script in `scripts/gentest/` and re-generate them. This can happen after a refactor. It can be helpful to commit the updated tests in a dedicated commit so that they can be easier to ignore during review.
 
 To add a new test case add another HTML file to `/test_fixtures` following the current tests as a template for new tests.

RELEASES.md

@@ -6,6 +6,8 @@
 
 - Added `taffy::error::InvalidChild` Error type
 - `taffy::node::Taffy.new_leaf()` which allows the creation of new leaf-nodes without having to supply a measure function
+- Builder methods are now `const` where possible
+  - Several convenenience constants have been defined: notably `FlexboxLayout::DEFAULT`
 
 ### 0.2.0 Changed
 
@@ -24,6 +26,7 @@
 - `taffy::error::InvalidChild` is now `taffy::error::TaffyError`
 - `taffy::error::InvalidNode` has been removed and is now just a branch on the `TaffyError` enum
 - `taffy::forest::Forest` has been merged into `taffy::node::Taffy` for a performance boost up to 90%
+- `Size::undefined()` has been removed, use `Size::NONE` instead.
 
 ### 0.2.0 Fixed
 

benches/complex.rs

@@ -107,7 +107,7 @@ fn taffy_benchmarks(c: &mut Criterion) {
         b.iter(|| {
             let mut taffy = taffy::node::Taffy::new();
             let root = build_deep_hierarchy(&mut taffy);
-            taffy.compute_layout(root, taffy::geometry::Size::undefined()).unwrap()
+            taffy.compute_layout(root, taffy::geometry::Size::NONE).unwrap()
         })
     });
 
@@ -117,7 +117,7 @@ fn taffy_benchmarks(c: &mut Criterion) {
 
         b.iter(|| {
             taffy.mark_dirty(root).unwrap();
-            taffy.compute_layout(root, taffy::geometry::Size::undefined()).unwrap()
+            taffy.compute_layout(root, taffy::geometry::Size::NONE).unwrap()
         })
     });
 }