Add `lto` option to profiles.

Closes #759.

I'm not sure if I did this right!
Hmm. Is it possible for a `bin` to be a non-root? If not, then would it be sufficient to just check if the thing being built is a `bin`? (If so, then `lto = true` always inserts `-C lto`, and otherwise, the value of `lto` is irrelevant.)