Free `default()` function to augment `Default::default()`

[ilya-bobyr]

It feels a bit redundant to have to say Default::default() every time I need a new value of a type that has a Default instance.
Especially so, compared to Haskell, where the same functionality is called def.
Providing a free default() function that forwards to Default::default() seems to improve the situation.
The trait is still there, so if someone wants to be explicit and to say Default::default() - it still works, but if imported as std::default::default;, then the free function reduces typing and visual noise.

[ilya-bobyr]

Here is a pull request: #73001