Unify treatment of "flags"

[molpopgen]

Currently, we treat each flag type separately using bitflags!. This approach is okay, but arguably too verbose. We also are not doing anything useful with "node flags", accepting a bare u32 there, making the API inconsistent.

Since node flags allows extra flags specified by the client, we should define NodeFlags and a corresponding From<tsk_flags_t>.

For other flags types, we are not checking for the presence of invalid flags (X::all().contains(y)), which we arguably should be doing.

Further/finally, we are still leaking the C typedef tsk_flags_t into rust via the crate prelude. This should, "of course", be a newtype.

And, speaking of newtypes, all the existing flags types should be repr(transparent) themselves, so that they really are tsk_flags_t at the ABI level.

[molpopgen]

For other flags types, we are not checking for the presence of invalid flags (X::all().contains(y)), which we arguably should be doing.

Rather than be this strict, we'll simply strip off invalid bits via From and add a valid() -> bool function to the impl.