Misc cleanup (#3381)

* Give credit to recent big-time contributors in the main README.md

* Better named profiling scopes

* Document everything in memory.rs

* Better doc-strings

* Add a section about dependencies to the main README.md

* Improve egui_extras docs

* fix typos

crates/egui_extras/src/datepicker/button.rs

@@ -7,6 +7,7 @@ pub(crate) struct DatePickerButtonState {
     pub picker_visible: bool,
 }
 
+/// Shows a date, and will open a date picker popup when clicked.
 pub struct DatePickerButton<'a> {
     selection: &'a mut NaiveDate,
     id_source: Option<&'a str>,