Function simple_key 

fn simple_key(
    tokens: &mut TokenSlice<'_, Token>,
    invalid_description: &'static str,
    receiver: &mut dyn EventReceiver,
    error: &mut dyn ErrorSink,
)

Parse a TOML simple key

;; Key-Value pairs

simple-key = quoted-key / unquoted-key

quoted-key = basic-string / literal-string