Rollup merge of #34305 - Aaronepower:master, r=alexcrichton
Rollup merge of #34305 - Aaronepower:master, r=alexcrichton

Added Default trait for Cow.

Adds a default implementation for Cow. Which is the Owned's default.

GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust