Auto merge of #35054 - pwoolcoc:stringfromchars, r=brson

implement `From<Vec<char>>` and `From<&'a [char]>` for `String`

Though there are ways to convert a slice or vec of chars into a string,
it would be nice to be able to just do `String::from(&['a', 'b', 'c'])`,
so this PR implements `From<Vec<char>>` and `From<&'a [char]>` for
String.