Update TODOs and safety comments once rust-lang/reference#1392 is stable

[joshlf]

rust-lang/reference#1392 added bit validity guarantees for various types. Once that text has been included in a stable release, we can update various TODOs and safety comments which depended on it or on rust-lang/reference#1291.

EDIT: This is implemented in #462, but that PR can't be merged until the new reference text is included in a stable release.