Add is_whitespace and is_alphanumeric to str.
Add is_whitespace and is_alphanumeric to str.

The other methods from `UnicodeStr` are already stable inherent
methods on str, but these have not been included.

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