std: Switch string::ParseError to an empty enum
std: Switch string::ParseError to an empty enum

It can never be instantiated, so signify this by having it actually be an empty
`enum`.

cc #27734

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