The conversion of an integer to a string was put in very early to bootstrap formatted
printing, if I remember correctly. It's odd, though, and no longer necessary, if it ever
was. It also causes inconsistencies, since string(0xD800) cannot produce the UTF-8
encoding for that code point (by definition, surrogates are not legal in UTF-8) so must
produce something else. We chose the "\uFFFD" since that's the only reasonable
option, but that means:

1) the result isn't obvious
2) string(0xD800) and "\uD800" do different things: the former produces the
UTF-8 for U+FFFD while the latter is statically rejected.

I propose that, in some remote future, we eliminate this conversion from the language.