Implement FromIterator on the flags struct.
Implement FromIterator on the flags struct.

I corrected some outdated information about the derived trait
implementations while I was at it.

phil-opp/bitflags,astocko/bitflags,rustyhorde/blastfig,RustOS-Fork-Holding-Ground/bitflags,rustyhorde/blastfig,rust-lang/bitflags,bitflags/bitflags