Auto merge of #45369 - fintelia:patch-1, r=BurntSushi
Auto merge of #45369 - fintelia:patch-1, r=BurntSushi

Implement is_empty() for BufReader

Simple implementation of `is_empty` method for BufReader so it is possible to tell whether there is any data in its buffer.

I didn't know correct stability annotation to place on the function. Presumably there is no reason to place this feature behind a feature flag, but I wasn't sure how to tag it as an unstable feature without that.

CC: #45323

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