libcore: Export core::from_str::FromStr from core::prelude
Remove extra `#[cfg(stage0)]`
auto merge of #6185 : gifnksm/rust/prelude-from_str, r=graydon

`core::prelude`  re-exports `core::to_str::ToStr`, but doesn't re-export `core::from_str::FromStr`.
That is inconsistent.